Driver circuitry and operation

ABSTRACT

This application relates to methods and apparatus for driving a transducer with switching drivers. A switching driver has first and second supply node for receiving supply voltages and includes an output bridge stage, a capacitor and a network of switches. The network of switches is operable in different switch states to provide different switching voltages to the output bridge stage. A controller is configured to control the switch state of the network of switches and a duty cycle of output switches of the output bridge stage based on an input signal to generate an output signal for driving the transducer.

FIELD OF DISCLOSURE

The field of representative embodiments of this disclosure relates tomethods, apparatus and/or implementations concerning or relating todriver circuits, and in particular to switching driver circuits as maybe used to drive a transducer.

BACKGROUND

Many electronic devices include transducer driver circuitry for drivinga transducer with a suitable driving signal, for instance for driving anaudio output transducer of the host device or a connected accessory,with an audio driving signal.

FIG. 1 is a schematic illustration of circuitry for driving atransducer. As shown generally at 100 in FIG. 1, a driver 102 mayreceive an input signal Sin and generate a corresponding drive signalfor driving the transducer. The input signal Sin may, for example, be aninput audio or ultrasonic signal or haptic waveform or the like and maybe received from upstream circuitry (not shown) such as upstreamamplification and/or signal processing circuitry. The driver 102 drivesthe transducer 104 with the drive signal based on the input signal Sin.Where the transducer is an output transducer the driver 102 drives thetransducer 104 to cause the transducer to produce a desired output, e.g.an audible, ultrasonic or haptic output based on the input signal Sin.

In some applications the driver 102 may include a switching amplifierstage, e.g. a class-D amplifier stage or the like, for generating thedrive signal. Switching amplifier stages can be relatively powerefficient and thus can be advantageously used in some applications. Aswitching amplifier stage generally operates to switch an output nodebetween defined high and low switching voltages, with a duty cycle thatprovides a desired average output voltage over the course of the dutycycle for the drive signal. To provide a desired output voltage range,the switching amplitude may be switched between the peak high and lowvoltages of the desired output range, for instance between a high-sidevoltage VH and ground.

FIG. 1 illustrates an example of a single-ended driver, but it will beunderstood that in some implementations a transducer may be driven in abridge-tied-load configuration.

FIG. 2 illustrates an example of driver circuitry 200 for driving abridge-tied-load (BTL). FIG. 2 illustrates that each side of the load104 is connected to a respective driver 102-1 and 102-2. The output node201 of each driver 102-1 and 102-2 is switched between a high-sidevoltage VH and a low-side voltage VL, for example between a positivesupply voltage and ground, with controlled duty cycles so as to controlthe voltage across the load 104. FIG. 2 illustrates that each of thedrivers 102-1 and 102-2 comprise switches 202 a and 202 b, which may betypically implemented as MOSFETs, for selectively connecting the outputnode 201 of the driver to the high-side voltage or the low-side voltage.Modulators 203-1 and 203-2 control the duty-cycle of the switching ofthe switches 202 a and 202 b of the respective driver 102-1 and 102-2based on the input signal. The modulators 105 may generate PWM or PDMswitching signals based on the input signal as will be understood by oneskilled in the art. The drivers 102-1 and 102-2 can thus be seen asgenerating respective first and second drive signals which arecomponents of a differential driving signal for driving the load.

In at least some applications, for example for driving somepiezoelectric transducers, it may be desirable to generate drive signalswith relatively high amplitudes, for example with a peak-to-peak voltagerange of the order of tens of volts or so. This may therefore typicallyrequire the input voltage for the driver 102, i.e. the voltagedifference between VH and VL, to be relatively high to provide therequired output range.

For instance, piezoelectric or piezo transducers or ceramic transducersare increasingly being proposed for use in some applications, forinstance for audio, ultrasonic or haptics output, and may be consideredas an alternative to conventional cone and voice-coil type speaker orresonant actuators or the like. Piezoelectric transducers may beadvantageous in some applications, especially for portable electronicsdevices such as mobile telephones, laptop and tablet computers and thelike, due to their thin form factor, which may be beneficial in meetingthe demand for increasing functionality in such devices withoutsignificantly increasing their size. Piezoelectric transducers are alsoincreasingly finding application as transducers for ultrasonic andrange-finding systems. Piezoelectric transducers may also be used asinput transducers or sensors in some applications.

Using such high input voltages can, however, result in relatively largevoltages stresses across the switches of the driver, which may requirethe use of devices with high voltage tolerances, which may not bepractical for some applications, or which may add to the cost of thecircuitry.

Using such high input voltages as the switching voltages for theswitching drivers may also, in some implementations, require the use ofcomponents such as inductors with a relatively high inductances so as toavoid large ripple current due to the switching, which may bedisadvantageous in some implementations.

SUMMARY

Embodiments of the present disclosure relate to improved drivingcircuits.

According to an aspect of the disclosure there is provided a switchingdriver for driving a transducer comprising: first and second supplynodes for connection to first and second voltage supplies defining aninput voltage; an output bridge stage comprising a first output switchconnected between a first switching voltage node and an output node anda second output switch connected between a second switching voltage nodeand the output node; a capacitor; and a network of switches connectingsaid first and second supply nodes with said first and second switchingvoltage nodes and said capacitor. The network of switches may beoperable in different switch states to provide different voltages at thefirst and second switching voltage nodes. The switch states may comprisea first switch state in which the capacitor is connected between thefirst and second supply nodes to be charged to the input voltage and thefirst and second switching voltage nodes are coupled to the first andsecond supply nodes respectively; and a second switch state in which thesecond switching voltage node is connected to a voltage different tothat at the second supply node and the capacitor is connected betweenthe second switching voltage node and the first switching voltage nodeto provide a boosted voltage at the first switching node. The driver mayalso comprise a controller configured to control the switch state of thenetwork of switches and a duty cycle of the first and second outputswitches of the output bridge stage based on an input signal to generatean output signal at the output node for driving the transducer.

In some examples, in the second switch state, the second switchingvoltage node is connected to the first supply node. The network ofswitches may be further operable in a third switch state in which thefirst switching voltage node is connected to the second supply node andsaid capacitor is connected between the second supply node and thesecond switching voltage node to provide a boosted voltage at the secondswitching node. The first voltage supply may be more positive that thesecond voltage supply and the controller may be configured to operate:in the first switch state in a first mode of operation to provide adrive signal at the output node in a range between the first and secondvoltage supplies; in the second switch state in second mode of operationto provide a drive signal at the output node in a range between thefirst voltage supply and the first voltage supply boosted positively bythe input voltage; and in the third switch state in a third mode ofoperation to provide a drive signal at the output node in a rangebetween the second voltage supply and the second voltage supply boostednegatively by the input voltage.

In some examples, the network of switches may be configured such that,in use, a voltage difference across any of the switches of the networkof switches and the first and second output switches is notsubstantially greater in magnitude than the input voltage.

The network of switches may be configured such a first electrode of thecapacitor can be selective connected to either of the first or secondsupply nodes. The network of switches may be configured such that asecond electrode of the capacitor can be selectively connected to eitherof the first or second supply nodes. In some examples, the capacitor maybe connected between the first and second switching voltage nodes inparallel with the output bridge stage. The network of switches maycomprises: a first switch connecting the first supply node to a firstsupply select node; a second switch connecting the second supply node tothe first supply select node; a third switch connecting the first supplyselect node to the first switching voltage node; a fourth switchconnecting the first supply node to a second supply select node; a fifthswitch connecting the second supply node to the second supply selectnode; and a sixth switch connecting the second supply select node to thesecond switching voltage node.

The controller may be configured such that, when operating in the secondswitch state: the fourth switch and sixth switch are closed, with thefifth switch open, to connect the first supply node to the secondswitching voltage node; and the second and third switches are open, withthe first switch closed, so as to disconnect the first switching voltagenode from the first and second supply nodes and to limit the voltagedifference across any of the first to third switches to be notsubstantially greater than the input voltage.

The capacitor may have a capacitance which is large enough to storesufficient charge to supply the transducer through a cycle of the inputsignal.

In some examples, the capacitor is a first capacitor, and the switchingdriver may further comprises a second capacitor. In at least one of thefirst and second switch states, the second capacitor may be connectedbetween the first and second voltage supplies to be charged to the inputvoltage. In the second switch state, the second switching voltage nodemay be connected to the first supply node. The switch network may befurther operable in a third switch state in which the second capacitoris connected between the first supply node and the second switchingvoltage node to provide a boosted voltage at the second switching nodeand the first capacitor is connected between the second switchingvoltage node and the first switching voltage node to provide a furtherboosted voltage at the first switching voltage node. The first capacitormay be connected between the first and second switching voltage nodes inparallel with the output bridge stage.

In some examples, at least one of the switches of the network ofswitches and the first and second output switches comprises an NMOStransistor where at least part of the NMOS transistor is formed withinan N-well in a substrate and wherein the switching driver is configuredsuch that the N-well of the NMOS transistor is, in use, driven with avoltage based on the voltages at the first and second switching voltagenodes.

In some examples, the switching driver may be a first switching driverand a switching driver circuit may further comprise a second switchingdriver, the first and second switching drivers being configured to drivethe transducer in a bridge-tied load configuration. The second switchingdriver may comprise an output bridge stage, a capacitor and a network ofswitches operable in the same way as the first switching driver. Thecontroller may be configured to control the switch state of the networkof switches and duty cycles of the output bridge stages of both thefirst and second switching drivers based on an input signal to generatea differential output signal.

In some examples a switching driver circuit may comprising the switchingdriver of any of the embodiments described herein and a DC-DC converterconfigured to receive at least one input voltage supply and to generateat least one of said first and second voltage supplies.

In some examples an inductor may be connected to the output node forconnection in series with the transducer.

Embodiments also relate to a switching driver circuit comprising aswitching driver of any of the embodiments and the transducer. Thetransducer may be at least one of an audio output transducer and ahaptic output transducer. The transducer may be a piezoelectric orceramic transducer. Embodiments also relate to an electronic devicecomprising such a switching driver.

In another aspect there is provided a switching driver for driving atransducer based on an input signal comprising: first and second supplynodes for connection to first and second voltage supplies defining aninput voltage; an output bridge stage comprising a first output switchconnected between a first switching voltage node and an output node anda second output switch connected between a second switching voltage nodeand the output node; a capacitor connected between said first and secondswitching voltage nodes in parallel with the output bridge stage; and anetwork of switches connecting said first and second supply nodes withsaid first and second switching voltage nodes, the network of switchesbeing operable in different switch states. In a first switch, the firstand second switching voltage nodes may be coupled to the first andsecond supply nodes respectively and the capacitor is charged to theinput voltage. In a second switch state, the second switching voltagenode may be connected to a voltage different to that at the secondsupply node and the capacitor is connected between the second switchingvoltage node and the first switching voltage node to provide a boostedvoltage at the first switching node.

Aspects also relate to a switching driver for driving a transducercomprising: first and second supply nodes for connection to first andsecond voltage supplies defining an input voltage; an output bridgestage for selectively connecting an output node to first or secondswitching voltage nodes with a controlled duty cycle; a capacitorconnected between said first and second switching voltage nodes inparallel with the output bridge stage; and a network of switchesconnecting said first and second supply nodes with said first and secondswitching voltage nodes, wherein the network of switches is operablesuch that a first electrode of said capacitor can be selectivelyconnected to either of the first and second supply nodes.

In a yet further aspect, there is provided a driver circuit for drivinga transducer based on an input signal comprising: first and secondswitching drivers with respective driver output nodes for driving saidtransducer in a bridge-tied-load configuration, each of the first andsecond switching drivers comprising a respective output stage forcontrollably switching the respective driver output node between highand low switching voltages with a controlled duty cycle. Each of thefirst and second switching drivers is operable in a plurality ofdifferent driver modes, wherein the switching voltages are different insaid different driver modes; and a controller controls the driver modeof operation and the duty cycle of each of the first and secondswitching drivers based on the input signal. The controller isconfigured to control the duty cycles of the first and second switchingdrivers within defined minimum and maximum limits of duty cycles. Thecontroller is configured to transition between different driver modes ofoperation when the duty cycle of at least one of the first and secondswitching drivers reaches the maximum or minimum duty cycle limit andthe controller is configured to implement the transition by changing thedriver mode of one of the first and second switching drivers at saidmaximum or minimum limit of duty cycle and vary the duty cycle of thatone of the first and second switching drivers to the other limit of dutycycle whilst maintaining the driver mode of the other of the first andsecond switching drivers and applying a variation in duty cycle tomaintain a magnitude of a differential component of the first and seconddriver signal.

In some examples, the maximum limit of duty cycle may be 95% or less.The minimum limit of duty cycle may be 5% of greater.

In some examples the difference between the switching voltages may bethe same in each of the different driver modes.

In some examples, each of the first and second switching drivers may beoperable in: a first mode in which the switching voltages are a firstvoltage V1 and a second voltage V2, where the first voltage is morepositive than the second voltage; and at least one of: a second mode inwhich the switching voltages are the first voltage V1 and a voltagewhich is more positive than the first voltage V1; and a third mode inwhich the switching voltages are the second voltage V2 and a voltagewhich is more negative than the second voltage V2. Each of the first andsecond switching drivers may have first and second supply nodes forreceiving first and second supply voltages, and the first and secondswitching drivers may be configured such that the first voltage V1 andthe second voltage V2 are equal to the supply voltages received at therespective first and second supply nodes. The controller may beconfigured to control the driver modes of the first and second switchingdrivers to provide a plurality of BTL modes. The BTL modes may comprisea low-signal mode in which both the first and second switching driversoperate in the first driver mode; an intermediate-signal mode in whichone of the first and second switching drivers operate in the firstdriver mode and the other one of first and second switching driversoperates in either the second driver mode or the third driver mode;and/or a high-signal level mode in which one of the first and secondswitching drivers operates in the second driver mode and the other oneof first and second switching drivers operates in the third driver mode.The controller may be configured such that, in each of the BTL modes ofoperation, the controller controls the duty cycles of the first andsecond switching drivers so that a common-mode component of first andsecond drive signals at the respective driver output nodes of the firstand second switching drivers does not substantially vary with signallevel. The controller may be configured such that the common-modecomponent of first and second drive signals when operating in thelow-signal mode is substantially the same as the common-mode componentof the first and second drive signals when operating in the high-signalmode, but different to the common-mode component of first and seconddrive signals when operating in the intermediate-signal mode.

In some examples, the controller may be configured to control the dutycycles of the first and second switching drivers before and after atransition in mode such that a common-mode component of the voltageacross the transducer differs before and after the transition.

Each of the first and second switching drivers may comprise a variableboost stage selectively operable to provide voltage boosting to providea voltage for use as a switching voltage in at least one of thedifferent driver modes. Each of the first and second switching driversmay comprise first and second supply nodes for receiving first andsecond supply voltages and the variable boost stage of each of the firstand second switching drivers may comprise at least one capacitor and anetwork of switches for connecting the first and second supply nodeswith the capacitor and the respective output stage. The network ofswitches may be operable in one switch state in which the at least onecapacitor is connected between the first and second supply nodes to becharged to the input voltage and in at least one switch state where theat least one capacitor is connected in series with one of the first andsecond supply nodes to provide a boosted voltage as one of the switchingvoltages.

In some examples, the driver circuit may further comprise at least afirst inductor connected to the driver output node of the firstswitching driver for connection in series with the transducer.

In a further aspect, there is provided a driver circuit for driving atransducer based on an input signal comprising: first and secondswitching drivers configured to drive the transducer in abridge-tied-load configuration; wherein each of the first and secondswitching drivers comprise an output stage for controllably switching adriver output node two switching voltages with a controlled duty cycleand wherein each of the first and second switching drivers is operablein a plurality of different driver modes where the switching voltagesare different in the different driver modes. A controller may controlthe driver mode of operation and duty cycle of each of the first andsecond switching drivers based on the input signal, wherein thecontroller is configured to controllable vary the duty cycle within aminimum duty cycle limit greater than 0% and a maximum duty cycle limitless than 100% and to transition between driver modes when one of theminimum or maximum duty cycle limits is reached. The controller may beconfigured such that during any mode transition, the driver mode of justone of the first and second switching drivers is changed.

In a further aspect, there is provided a driver circuit for driving atransducer based on an input comprising: first and second switchingdrivers for generating respective first and second drive signals atfirst and second output nodes for driving the transducer in a bridgetied load configuration, wherein each of the first and second switchingdrivers is configured to controllably switch the respective first orsecond output node between two switching voltages with a controlled dutycycle; and wherein the first and second switching drivers are configuredsuch that the two switching voltages are controllably variable toprovide different BTL modes of operation. The driver circuit may beconfigured so as to transition between modes by: controllably varyingthe switching voltages for one of the first and second switching driverswhilst switching the duty cycle of that driver from one of a maximum orminimum limit of duty cycle to the other or the maximum or minimumlimit; whilst maintaining the switching voltages for the other of thefirst and second switching drivers and applying a corresponding changein duty cycle to maintain a differential voltage.

In a further aspect, there is provided a driver circuit for driving atransducer based on an input signal comprising: first and second supplynodes for receiving an input voltage; first and second switching driverswith respective driver output nodes for driving said transducer in abridge-tied-load configuration, each of the first and second switchingdrivers comprising a respective output stage for controllably switchingthe respective driver output node between high and low switchingvoltages with a controlled duty cycle to provide respective first andsecond drive signals. Each of the first and second switching drivers maybe operable such that the high and low switching voltages can becontrollably varied in at least three different driver modes. In a firstdriver mode the high switching voltage is at a first voltage level V1and the low switching voltage is at a second voltage level V2, and thefirst and second voltage levels differ by an amount equal to the inputvoltage. In a second driver mode the low switching voltage is at thefirst voltage level V1 and the high switching voltage is higher than thefirst voltage level V1 by an amount equal to the input voltage. In athird driver mode the high switching voltage is at the second voltagelevel V2 and the low switching voltage is lower than the second voltagelevel V2 by an amount equal to the input voltage. A controller maycontrol the driver mode and duty cycle of each of the first and secondswitching drivers based on the input signal, wherein the controller isoperable in a plurality of BTL modes of operation comprising: alow-signal BTL mode to operate both the first and second switchingdrivers in the first driver mode; an intermediate signal BTL mode tooperate one of the first and second switching drivers in the firstdriver mode and the other of first and second switching drivers in oneof the second or third driver mode; and a high-signal BTL mode tooperate one of the first and second switching drivers in the seconddriver mode and the other of first and second switching drivers in thethird driver mode.

Embodiments also relate to a driver circuit of any of the embodimentsdescribed herein comprising the transducer. The transducer may be atleast one of an audio output transducer and a haptic output transducer.The transducer may be a piezoelectric or ceramic transducer.

Embodiments also relate to an electronic device comprising a drivercircuit of any of the embodiments described herein.

It should be noted that, unless expressly indicated to the contraryherein or otherwise clearly incompatible, then any feature describedherein may be implemented in combination with any one or more otherdescribed features.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of examples of the present disclosure, and toshow more clearly how the examples may be carried into effect, referencewill now be made, by way of example only, to the following drawings inwhich:

FIG. 1 illustrates one example of a single-ended driving circuit fordriving a load;

FIG. 2 illustrates one example of a driver circuit for driving a load ina bridge-tied-load configuration;

FIG. 3 illustrates example output waveforms of a switching driveraccording to an embodiment;

FIG. 4 illustrates one example of a driver circuit according to anembodiment;

FIG. 5 illustrates one example of the driver circuit of figure in moredetail;

FIG. 6 illustrates one example of a MOSFET implementation of a switchingdriver;

FIG. 7 illustrates one example of NMOS devices suitable for use in theswitching driver of FIG. 6;

FIG. 8 illustrates one example of suitable operating voltages fordifferent modes of operation;

FIG. 9 illustrates an example of one example of NMOS and PMOS devicessuitable for use in the switching driver of FIG. 6;

FIG. 10 illustrates another example of an implementation of a switchingdriver;

FIG. 11 illustrates NMOS devices suitable for use in the switchingdriver of FIG. 10;

FIG. 12 illustrates a driver circuit in a bridge-tied-loadconfiguration;

FIG. 13 illustrates example waveforms for the drivers on each side ofthe load and the resultant differential voltage for a bridge-tied loadconfiguration;

FIG. 14 illustrates how the duty-cycles of the switching drivers on eachside of the load may vary for the differential waveform illustrated inFIG. 13;

FIG. 15 illustrates example waveforms for a mode transition for asingle-ended driver;

FIG. 16 illustrates one example of waveforms for a driver circuit fordriving a bridge-tied-load;

FIG. 17 illustrates examples waveforms across a mode transition; and

FIG. 18 illustrates one example illustrates duty cycle of the switchingdrivers may be controllably varied to manage discontinuities.

DETAILED DESCRIPTION

The description below sets forth example embodiments according to thisdisclosure. Further example embodiments and implementations will beapparent to those having ordinary skill in the art. Further, thosehaving ordinary skill in the art will recognize that various equivalenttechniques may be applied in lieu of, or in conjunction with, theembodiments discussed below, and all such equivalents should be deemedas being encompassed by the present disclosure.

Embodiments of the disclosure relate to driver circuitry for driving atransducer and, in particular, to driver circuitry suitable for drivinga reactive load such as a piezoelectric transducer, although embodimentsmay be implemented to drive other types of transducer. Embodiments ofthe disclosure also relate to methods of operation of driver circuitry.

At least some embodiments of the present disclosure relate to switchingdrivers for generating a drive signal at a driver output node. Theswitching driver is operable in a plurality of different driveroperating modes, wherein, in each of the different driver operatingmodes, the output node is switched between two switching voltages, wherethe switching voltages are different in the different modes ofoperation. Thus, in a given driver mode of operation, the switchingdriver operates to switch the driver output node between the relevantswitching voltages with a controlled duty cycle so as to provide thedrive signal with an average voltage (over the course of a cycle period)within a voltage range defined by the switching voltages. However, in adifferent driver mode of operation, the switching voltages are differentso as to provide, in that driver mode, a different voltage range for thedrive signal. The overall output voltage range for the switching drivermay thus be defined by the different driver modes of operation, and eachindividual driver mode of operation may provide only part of the overalloutput voltage range, that is, the voltage range between the twoswitching voltages in a given mode forms only a part or a subset of theoverall output voltage range.

The switching driver thus switches between two defined switchingvoltages with a controlled duty cycle to provide the drive signal at thedriver output node with a desired average output voltage. In use, theaverage output voltage can vary within a defined voltage range between apeak high voltage VH and a peak low voltage VL. However, rather thanswitch a driver output node between these peak high and low voltagelevels of the output range, as would be conventional for the driversdiscussed with respect to FIGS. 1 and 2, the switching driver ofembodiments of the disclosure switches between two switching voltageswhich form a subset, or only part, of the full output range between VHand VL. Thus, in each driver mode of operation, the output node isswitched between two switching voltages that differ from one another byless than the full output range, but the full output range can beprovided by varying the driver mode of operation, as necessary.

In effect, the switching driver may be considered to operate withvariable voltage rails, where the voltage rails are controllably variedto provide different operating ranges in the different operating modes,where each of the operating ranges is only part of the of full outputrange of the driver.

Thus, a given peak-to-peak voltage variation for the driver signal canbe achieved using switching voltages with a voltage difference which islower than the peak-to-peak variation. Using switching voltages with alower voltage difference can be beneficial in terms of reduced switchinglosses and reduced radiated emissions, as well as reducing therequirement for a significant output inductance.

FIG. 3 illustrates this principle. FIG. 3 illustrates the switchingwaveforms at an output node of a switching driver according to oneexample and the resulting average voltage 301 over a duty cycle, i.e.the desired voltage of the drive signal output from the switchingdriver. It should be noted that FIG. 3 illustrates the switchingwaveforms for the driver signal generated at one driver output node,i.e. as would be applied to just one side of the load.

In this example, the drive signal may vary within a full output rangebetween a peak low voltage VL and a peak high voltage VH. In thisexample, however, the switching driver is operable in different drivermodes. In one mode of operation an output node of the switching drivermay be switched between the peak high voltage VH and a firstintermediate voltage V1. In another mode of operation, the output nodemay be switched between the first intermediate voltage V1 and a secondintermediate voltage V2. In a further mode of operation, the output nodemay be switched between the second intermediate voltage V2 and the peaklow voltage VL.

To generate a drive signal with a voltage in the range between the peakhigh voltage VH and the intermediate voltage V1, the output stage mayoperate in the mode that switches between VH and V1. For a drive voltagewhich is between the first intermediate voltage V1 and the secondintermediate voltage, the output node may be switched between V1 and V2,and if the desired voltage for the drive signal is between V2 and thepeak low voltage VL, the switching driver may operate in the mode toswitch the voltage at the output node between V2 and the peak lowvoltage VL. In each case, the duty cycle is controlled appropriately toprovide the desired average voltage.

FIG. 3 illustrates that the full driver output range between VL and VHis provided by three different driver operating modes. However, in otherembodiments, there may be a different number of driver operating modesacross a full output range of the switching output stage, for instancein some embodiments there may be just two driver operating modes orthere may be more than three driver operating modes. The voltage rangesfor the driver operating modes, defined by the switching voltages, maybe defined so that the respective voltage ranges of the driver operatingmodes are contiguous and non-overlapping and collectively cover thewhole of the full output range of the switching driver. In otherembodiments however it may be advantageous to have some overlap betweenthe output voltage ranges in the different driver operating modes. Insome embodiments the magnitude of the voltage range of each of thedriver operating modes, i.e. the voltage difference between the relevanttwo switching voltages: VL and V2, V2 and V1, or V1 and VH; may be thesame as one another.

It will be understood that the reference to a voltage range, for theswitching driver in a given operating mode, refers to the voltage of thedrive signal at the driver output node (in terms of the average voltageover the course of a switching cycle) in that driver operating mode. Theactual voltage that is applied across the load will, of course, dependon the voltage on the other side of the load, e.g. a defined DC voltagefor a single-ended configuration or the voltage of a second drive signalon the other side of the load generated by another driver for a BTLconfiguration. The output signal range, for the output signal appliedacross the load, may therefore be different to the driver voltage range.For instance with reference to FIG. 3, if a defined DC voltage Vdc onthe other side of the load was set to be a midpoint voltage between V1and V2, then the switched driver may operate in the mode with switchingvoltages V1 and V2 for low output signal levels, up to a magnitude of(V1−V2)/2 and may swap to switching voltages V1 and VH for higherpositive output signal values (up to VH−Vdc) and swap to switchingvoltages VL and V2 for greater negative output signal values (down toVL−Vdc).

As used herein the term driver operating mode will thus refer to theoperation of a driver on one side of the load only, and the reference toa driver output voltage or output voltage range, or just voltage range,will refer to the voltage of a drive signal at the diver output node.For at least some BTL configurations, each side of the load may be seenas being driven by a respective driver, i.e. there are first and seconddrivers for driving both sides of the load. In some implementations, aswill be discussed in more detail below, both of the drivers of a BTLimplementation may be separately operable in different modes, in whichcase an overall BTL operating mode may be defined by the individualoperating modes of the individual drivers.

The switching driver may be configured to receive a high-side supplyvoltage and a low-side supply voltage, for instance a positive supplyvoltage and ground, which define an input voltage with a magnitude whichis significantly lower than the full peak-to-peak output voltage rangeof the switching driver. Operating in the different driver operatingmodes also means that the voltage difference between the switchingvoltages is significantly less than the full output range of theswitching driver, even when operating to provide a drive signal voltagenear the peak high output voltage VH. In this way, the maximum voltagestress across components of the switching driver can be kept to amagnitude which is lower than, and in some implementations significantlylower than, the peak-to-peak output voltage range of the switchingdriver. This can advantageously allow the use of components, e.g.transistors such as. FETs, with a voltage tolerance which may besignificantly lower than the peak-to-peak output voltage of the drivercircuit.

The switching driver may comprise a variable boost, or level-shiftingcircuit, such a charge pump, for providing some of the appropriateswitching voltages for operation in the selected driver mode. Thevariable boost circuit may selectively positively boost (i.e. increase)the high-side input voltage and/or negatively boost (i.e. lower) thelow-side input voltage to controllably vary at least one of theswitching voltages for a mode of operation.

FIG. 4 illustrates one example of driving circuitry 400 with a variableboosting switching driver 401. In the example of FIG. 4 the switchingdriver 401 comprises a variable boost stage 402 and a switching outputbridge stage 403. The variable boost stage 402 is configured to receivea high-side supply voltage VSUP and a low-side input supply voltage,which in this example is ground, which together define an input voltageVin, i.e. as the voltage difference between the high- and low-sidesupplies. The switching output bridge stage 403 receives high-side andlow-side-switching voltages VSH and VSL from the variable boost stageand switches a driver output node 404 between these two switchingvoltages with a controlled duty cycle to provide a desired averageoutput voltage. In the example of FIG. 4 the high-side supply voltageVSUP for the switching driver 401 is derived from a power supply voltageVPS, which may, for instance be a battery voltage. In someimplementations the power supply voltage VPS, may be regulated and/orlevel shifted to provide an appropriate supply voltage VSUP for theswitching driver, for instance there may be an initial boost stage 405operable to provide a defined boost to the received power supply voltageVPS.

The variable boost stage is operable to controllably vary the switchingvoltages VSH and VSL supplied to the output bridge stage 403 so as toprovide the different driver modes of operation. In one example, in onemode of operation, the supply voltages to the switching driver 401, i.e.the voltages VSUP and ground (0V), may be used as the switching voltagesVSH and VSL. In this mode of operation the supply voltages received bythe switching driver 401 are thus used as the switching voltages andsuch a mode of operation may be seen as an unboosted driver mode ofoperation as no voltage boosting is applied by the switching driver. Itwill be understood however, as described above, that at least one of thesupply voltages received by the switching driver may itself have beenboosted by an upstream component.

In one example, in another driver mode of operation, the supply voltageVSUP may be used as the low-side switching voltage VSL, with thevariable boost stage being operable to generate a positively boostedhigher voltage as the high-side switching voltage VSH. In one example,in another driver mode of operation, the low-side supply voltage, i.e.ground in this example, may be used as the high-side switching voltageVSH, with the variable boost stage being operable to generate anegatively boosted lower voltage as the low-side switching voltage VSL.

In some applications the load 104 could be a reactive load, such as apiezoelectric transducer. FIG. 4 also illustrates that there may be aseries inductance 406 in the load path, e.g. the load 104 may beconnected in series with an inductor 406. Especially for piezoelectrictransducers, the capacitive nature of such transducers means that it maygenerally be beneficial to include an inductor in series with thetransducer. The inductor 406 may help suppress the switching ripple atthe switching frequency, whilst allowing the current to flow for thesignal band of interest, e.g. at audio or ultrasonic frequencies.

FIG. 5 illustrates one example of the driver circuitry of FIG. 4 in moredetail and similar components are identified by the same references.

FIG. 5 illustrates that the output bridge stage 403 comprises twoswitching paths for selectively connecting the driver output node 404 toa first node N1 or a second node N2. Each switching path comprises arespective output switch SWO1 and SWO2. The variable boost stage 402 isconfigured to selectively control the voltages at the first and secondnodes N1 and N2 to enable the various modes of operation, and thus thefirst and second nodes N1 and N2 may be referred to as first and secondswitching voltage nodes.

The variable boost stage 402 comprises, in use, a capacitor 501 andnetwork of switches which are operable in a plurality of differentswitch states so as to provide different voltages at the switchingvoltage nodes N1 and N2. The network of switching paths is arranged suchthat the capacitor 501 can be charged to a desired voltage, which inthis case is the input voltage. The switch network is also arranged sothat the charged capacitor 501 can selectively provide voltage boostingto one of the first and second switching voltages nodes in one of thedriver modes of operation.

In the example of FIG. 5, the capacitor 501 is connected between thefirst and second variable boost nodes N1 and N2 and thus in parallelwith the output bridge stage 403. This arrangement means that thevoltage difference between the switching voltages corresponds to thevoltage on the capacitor 501.

In the example of FIG. 5, the network of switches is arranged to providea first set of switching paths SW1A, SW1B and SW1C and a second set ofswitching paths SW2A, SW2B and SW2C. Switching paths SW1A and SW1B areprovided to selectively connect a node N3 to the high-side input voltageor the low-side input voltage respectively, i.e. VBST and ground in thisexample. The node N3 can thus be seen as a variable supply node or asupply select node. Switching path SW1C selectively connects the node N3to the first variable boost node N1. Likewise, switching paths SW2A andSW2B selectively connect a node N4 to the high-side input voltage or thelow-side input voltage respectively and switching path SW1C selectivelyconnects the node N4, which can be seen as a second variable supply, orsupply select, node, to the second variable boost node N2. Thisarrangement means that a first electrode of the capacitor 501 can beselectively connected to either of the first or second input supplies.Likewise a second electrode of the capacitor 501 can be selectivelyconnected to either of the first or second input supplies.

It will be understood that the switching driver 401 may be implementedas an integrated circuit (IC), but in some embodiments the capacitor 501may not be an integrated component and may be a separate component whichis connected to the IC in use, i.e. the capacitor 501 may be off-chip.The capacitor 501 may therefore be connected between first and secondcapacitor nodes (not separately identified), which may be connected tosuitable contacts of an IC for connection to an external capacitor.

In use, the driver circuit may be selectively operable in threedifferent driver modes of operation, where the voltages at the switchingvoltage nodes N1 and N2, and hence the switching voltages for the outputstage 403 vary in each mode.

In a first mode of operation the switching voltages may be +VSUP andground. In a second mode of operation the switching voltages may be+2VSUP and +VSUP. In a third mode of operation the switching voltagesmay be ground and −VSUP. By swapping between the three modes ofoperation the switching driver circuit 400 can provide a peak-to-peakvoltage range of 3VSUP in magnitude, from −VSUP to +2VSUP.

To provide the first mode of operation the switch network of thevariable boost stage may be operable in a first switch state, in whichswitching paths SW1A and SW1C may be closed (with switching path SW1Bopen) to connect the high-side supply voltage VSUP to the firstswitching voltage node N1, and the switching paths SW2B and SW2C may beclosed (with switching path SW2A open) to connect the second switchingvoltage node N2 to the low-side supply voltage, i.e. ground in thisexample. This results in the switching voltage nodes N1 and N2 beingconnected to the respective high-side and low-side supply voltages.Whilst in this first state, the output bridge stage 403 can becontrolled so that switching paths SWO1 and SWO2 alternately connect thedriver output node 404 to first and second switching voltage nodes witha duty cycle controlled to provide the desired average output voltage inthe range 0V to +VSUP.

In this first state of the first mode of operation, the capacitor 501 isalso connected between the high-side and low-side supply voltages andthus is charged to the input voltage Vin for the variable boost stage,i.e. to +VSUP in this example.

As, in this first state of the first mode, the switching voltage nodesN1 and N2 are connected to the respective high-side and low-sidesupplies, the load current can be supplied from the input supply to thevariable boost stage.

In some implementations the switch network of the variable boost stage402 may additionally be selectively operable in an alternative switchstate to provide the same switching voltages as the first mode.Switching paths SW2B and SW1C may be closed (with switching path SW2Aopen) to connect the second switching voltage node N2 to ground, as inthe first state, but in this alternative switch state the switching pathSW1C may be open. In this case the switching voltage node N1 is isolatedform the supply voltages (other than the path via the capacitor 501),but the voltage on (previously charged) capacitor 501 will maintain thevoltage at the first switching voltage node N1 substantially equal toVSUP. In this alternative switch state, any load current can be suppliedfrom the capacitor 501 (when the output path SWO1 is closed).

It will be noted that, in this first mode of operation the maximumvoltage difference across any of switching paths, and thus any of theswitches of the switch network, is substantially equal to the magnitudeof the input voltage Vin, i.e. the voltage difference between thehigh-side and low-side supply voltages. The voltages at the switchingvoltage nodes N1 and N2 are +VSUP and ground respectively, whilst thevoltages at node N3 and N4 are VSUP and ground respectively (at least inthe first state—in the alternative state the voltage at node N3 may befloating or this node could be selectively connected to VSUP or groundby switching paths SW1 a or SW1B respectively).

To provide the second mode of operation, switching path SW2A is closed(with switching path SW2B open) to connect node N4 to the high-sidesupply voltage VSUP and switching path SW2C is closed to connect node N4to the second switching voltage node N2. In this state, the secondswitching voltage node N2 is thus substantially equal to the high-sidesupply voltage +VSUP. Switching path SW1C is open, and the voltage ofthe capacitor 501, which is charged to the input voltage Vin, in thiscase equal to +VSUP, positively boosts the voltage at the firstswitching voltage node N1 to +2VSUP. This driver mode can thus be seenas a positive boosted mode of operation.

In this second switch state the voltage at the first switching voltagenode N1 is boosted to +2VSUP (i.e. boosted above the high-side supplyvoltage VSUP by the voltage Vin on the capacitor 501), and thus thisnode is disconnected from the supply voltages. It will be noted that ifswitch SW1C were not present, this disconnection could be achieved byopening both switching paths SW1A and SW1B. However, in that case thevoltage at node N3 would be the same as that at the first switchingvoltage node N1, i.e. equal to +2VSUP and thus the voltage across switchSW1B would be equal to +2VSUP, i.e. this switch would be subjected to avoltage stress of twice the input voltage. The inclusion of switchingpath SW1C reduces the voltage stress. Switching path SW1C is open, todisconnect the first switching voltage node N1 from the supply selectnode but the switching path SW1A may be closed (with switching path SW1Bopen) to connect node N3 to the high-side supply voltage VSUP. Thislimits the voltage difference across the switch of the open switchingpath SW1C to a voltage of magnitude equal to the input voltage Vin(which in this case equals VSUP). Thus, the maximum voltage differenceacross any of the switching paths is again substantially equal to themagnitude of the input voltage.

Whilst the variable boost stage is in this second switch state, theoutput bridge stage 403 can be controlled so that the output switches ofthe switching paths SWO1 and SWO2 alternately connect the driver outputnode 404 to first and second switching voltage nodes with a duty cyclecontrolled to provide the desired average output voltage within therange +VSUP to +2VSUP.

When operating in this second driver mode, the load current will bedrawn from the capacitor 501. However, if the capacitance of thecapacitor 501 is relatively large and the load for the output stage is areactive load, the capacitor 501 can provide the charge needed with justone charge pumping cycle. In particular, when the output bridge stage403 drives an inductor 406 in the output path, this can enable losslessmovement of charge between the load and the capacitor 501 for chargerecovery.

It will be understood that for driving a transducer with a drive signalbased on an input signal, such as an audio signal, the required drivevoltage of the switched driver will vary with the input signal. Highoutput voltages, such as enabled in the second driver mode of operationmay only be required for relatively large amplitude drive signals andfor only part of the input signal cycle, when the input signal is nearits peak. Thus, in normal operation, the second driver mode may beexpected to be used for only parts of the signal cycle of the inputsignal, and for the switching driver circuit to be operating in thesecond driver mode of operation, e.g. in the range +VSUP to +2VSUP, thedrive signal output from the switching driver will have passed throughthe voltage range of the first mode, e.g. 0V to +VSUP. Thus prior tooperating in the second driver mode there will have been a period ofoperation in the first driver mode and thus the capacitor 501 will havebeen charged during such operation in the first driver mode.

The size of the capacitor 501 may thus be selected, based on thereactive load to be driven, such that a single charge of the capacitorprovides sufficient charge for the driving of the reactive load over thecourse of a signal cycle for the input signal.

It will be noted that in the second switch state the second switchingvoltage node is connected to a voltage other than the second supplyvoltage (e.g. ground) and, in the example of FIG. 5, this is the firstsupply voltage VSUP. However, in some embodiments a different definedvoltage, say 0.5VSUP for example, could be derived and supplied to thesecond switching voltage node in this second stage, in which case thecapacitor would boost the voltage at the first switching voltage node to1.5VSUP.

To provide the third mode of operation, the switch network of thevariable boost stage may be operable in a third switch state, in whichswitching paths SW1B and SW1C are closed (with switching path SW1A open)to connect the first switching voltage node to the low-side supplyvoltage, i.e. ground. The first switching voltage node N1 is thussubstantially equal to the low-side supply voltage, i.e. ground in thisexample. Switching path SW2C is open and the voltage of the capacitor502, which is charged to the input voltage, i.e. VSUP in this case,negatively boosts, or lowers, the voltage at the second switchingvoltage node N2 to −VSUP. To control the voltage stress across theswitch path SW2C, the switching path SW2B may be closed to cause thevoltage at node N4 to be equal to ground.

Whilst the switch network of the variable boost stage is in this thirdswitch state, the output bridge stage 403 can be controlled so that theoutput switches SWO1 and SWO2 alternately connect the driver output node404 to first and second switching voltage nodes with a duty cyclecontrolled to provide the desired average output voltage within therange −VSUP to 0V.

It will thus be understood that the variable boost stage 402 is operableto controllably vary the voltages at the switching voltage nodes N1 andN2 to provide different switching voltages for the output stage 403 inthe different driver modes of operation. One of switching voltages inthe second and third modes of operation is selectively boosted by thevoltage of the capacitor 501, which is charged during operation in thefirst mode by the input voltage, i.e. the voltage between the high-sidesupply voltage and the low-side supply voltage, VSUP and ground in thisexample. Each of the operating modes therefore involves switchingvoltages that differ from one another by a magnitude equal to the inputvoltage Vin for the variable boost stage, i.e. the difference betweenthe high-side and low-side supply voltages. In the embodiment of FIG. 5the capacitor is charged to the input voltage and as the capacitor isconnected in parallel with the output stage 501, the switching voltagesin a given mode of operation differ by an amount equal to the inputvoltage. In the embodiment of FIG. 5 one capacitor may be used toselectively provide both positive boosting or negative boosting asrequired.

It will also be clear that the maximum voltage stress across any of theindividual switching paths can be limited to be substantially equal tothe input voltage. Typically, each switching path may implemented with atransistor, e.g. a MOSFET, as a switch and this means that adrain-source voltage tolerance of the transistor need only be sufficientto withstand a voltage of magnitude equal to the input voltage supply tothe variable boost stage. Note that for correct operation of thetransistor it may be beneficial to implement at least some of thetransistor in doped wells which are driven with voltages based on theswitching voltages, as will be discussed in more detail below.

The input voltage for the switching driver 401 may thus define thevoltage tolerance required for the switches of the switch network of thevariable boost stage and also define the voltage range of the differentoperating modes, and hence the overall voltage range of the switchingdriver.

As noted above, in some examples, to provide an appropriate inputvoltage for the variable boost stage 402, the driver circuitry 400 maycomprise a first boost stage 405 for receiving a power supply voltageVPS and boosting the power supply voltage to provide at least one of thevoltage supplies for the switching driver 401. In this example the firstboost stage receives the power supply voltage VPS and boosts the powersupply voltage VPS to provide the supply voltage VSUP. The first booststage, if present, could be any suitable DC-DC converter with a voltageboost and FIG. 5 illustrates that the first boost stage may comprise aboost converter with an inductor 502, control switches SW3A and SW3B anda reservoir capacitor 503 coupled to maintain the output voltage VBST.

In some examples the power supply voltage VPS could be a battery voltageand thus could be of the order of a few volts, say a voltage at oraround 4.2V. In some examples this supply voltage could be boosted to asupply voltage VSUP of say 20V or so to provide the high-side supply tothe switching driver 401. The low-side voltage may, in some examples, beground, so the input voltage for the switching driver may be 20V. Forthe example illustrated in the FIG. 5, the switching driver may thus beoperable to generate a drive signal with a voltage range of 60V, from avoltage of −20V to a voltage of +40V, by selectively operating in one ofthe three different operating modes, a −20V to 0V mode, a 0V to +20Vmode and a +20V to 40V mode.

The use of a first voltage defined boost stage to generate a firstboosted voltage at a predetermined level which may then be selectivelyboosted by a variable boost stage represents one particular aspect ofthe present disclosure. In general, therefore, at least some embodimentsrelate to driver circuitry comprising a switching output stage 403 fordriving an output node 404 between two switching voltages with acontrolled duty cycle, where the two switching voltages are selectivelyvariable. A variable boost stage 402 may have first and second inputnodes configured to receive a first high-side voltage and a firstlow-side voltage respectively and provide the switching voltages atfirst and second variable boost nodes. The variable boosting stage maybe operably to selectively boost at least one of the first high-sidevoltage and the first low-side voltage to controllably vary theswitching voltages in different modes of operation. The driver includesa first defined boost stage 405 for a first defined boost stageconfigured to receive a supply voltage and boost the supply voltage to adefined voltage level to provide one of said first high-side voltage andfirst low-side voltage. The variable boost stage may be a switchedcapacitor variable boost stage, e.g. charge pump, which can allow forcharge recovery from the load.

As described above, each of the switches of the network of switches ofthe variable boost stage 402 and also the output switches of the outputbridge stage 403 may be implemented by a suitable transistor, e.g. asuitable MOSFET. FIG. 6 illustrates an example of a switch driver suchas described with respect to FIG. 5, wherein each of the switches of theswitching paths is implemented by a FET.

In one implementation, each of the FETs illustrated in FIG. 6 may be anNMOS FET device. At least part of the NMOS devices may be formed in deepN-wells as will be understood by one skilled in the art. FIG. 7illustrates schematically an example of two NMOS devices 701-1 and 701-2formed on a substrate 702. Each of the NMOS devices comprises source anddrain regions 703 and 704 formed in a bulk region 705, with a gateelectrode 706 for controlling channel conduction. The bulk region is 705is disposed within a deep N-well 707. Isolation regions 708 isolate theN-wells from neighbouring devices.

In use, the substrate 702 and isolation regions 708 will typically beheld at ground and the bulk 705 is typically coupled so as to be drivento the same voltage as the source 703. The deep N-wells 707 may bedriven with a voltage that varies according to the signal range on theoutput to avoid P-well to deep N-well breakdown or unwanted forwardbiasing of the deep N-well to the substrate or P-well. By driving thevoltages in this way, the transistor devices may be protected from highvoltages.

FIG. 8 illustrates a table showing one example how the N-wells of theNMOS switches of the switching driver 401 of FIG. 6 may be driven whenoperating in the different modes. The table illustrated in FIG. 8 liststhe switches of the switching driver for each of the operating modes,and in each case, the state of each of the switches, i.e. on, off orbeing operated with a controlled duty cycle (represented by DC), isindicated, along with the respective source/bulk and drain voltages andthe voltage to which the respective N-well may be driven.

In another implementation, the FETs illustrated in FIG. 6 may be amixture of NMOS and PMOS devices. For example, the switches of switchingpaths SW1A, SW1C, SWO1 and SW2A may be implemented as PMOS devices,whilst the switches of switching paths SW1B, SWO2, SW2B and SW2C may beimplemented as NMOS devices. As discussed, above the NMOS devices may beformed in deep N-wells. FIG. 9 illustrates schematically an example ofan NMOS device 701 and a PMOS device 901 formed on a substrate 902. TheNMOS device 701 may have the same structure as the NMOS device discussedwith reference to FIG. 7. The PMOS device also comprises a source region903 and drain region 904 in a bulk region 905, with a gate 906, but forthe PMOS device, as the bulk region is N-type there is no need for any Nwell.

In use, the switching driver 401 could be operated in the same threemodes as described above, and in operation the N-wells of the NMOSdevices could be driven in the same way as illustrated in FIG. 8, withthe bulk of the PMOS devices connected to be at the same voltage as thesource.

FIG. 10 illustrates another example of a switching driver circuit 401according to an embodiment. The embodiment of FIG. 10 includes an outputbridge stage 403 with first and second output switches SWO1 and SWO2connected between first and second switching voltage nodes N1 and N2, ina similar manner as described with respect to FIG. 5. The switchingdriver circuit 401 also includes a variable boost stage 402 forcontrolling the voltages at the switching voltage nodes N1 and N2, whichincludes a first capacitor 1001 connected in parallel with the outputbridge stage 403 between the first and second switching voltage nodes N1and N2 and a network of switches, but in this example the variable booststage also includes a second capacitor 1002.

The network of switches includes switching paths SW3A, SW3B and SW3C,which, together with the second capacitor 1002 effectively provide afirst variable boost sub-stage. The second capacitor 1002 is coupledbetween variable boost nodes N1-1 and N2-1 of the first sub-stage, andswitching paths SW3A and SW3B selectively connect variable boost nodesN1-1 and N2-1 to the high-side and low-side supply voltagesrespectively, in this example VSUP and ground. In use, with switchingpaths SW3A and SW3B closed (and switching path SW3C open), the variableboost nodes N1-1 and N2-1 of the first sub-stage are substantially equalto the respective high-side and low-side supply voltages and the secondcapacitor 1002 is charged to a voltage equal to the input voltage.Switching path SW3C selectively connects the high-side supply voltage tothe second variable boost node N2-1. With switching path SW3C closed(and the switching paths SW3A and SW3B open), the variable boost nodeN2-1 of the first stage is driven to be substantially equal to thehigh-side supply voltage, i.e. +VSUP in this example, and the voltage atvariable boost node N1-1 is positively boosted by the capacitor voltage,i.e. to +2VSUP in this example.

Switching paths SW4A, SW4B and SW4C together with the first capacitor1001, effectively provide a second variable boost sub-stage with thesame genera structure as the first sub-stage and which is operable ingenerally the same way. The second variable boost sub-stage receives thevoltages at nodes at the variable boost node N1-1 and N2-1 of the firstboost sub-stage as respective high-side and low-side voltages. Thesecond variable boost sub-stage is thus operable to either provide thesevoltages received from the first boost sub-stage as switching voltagesto the switching voltage nodes N1 and N2, or to provide the voltage fromnode N1-1 to the second switching voltage node N2, with the capacitor1001 providing further positive boosting for the voltage at the firstswitching voltage node N1.

The switching driver of FIG. 10 is thus operable in three differentoperating modes, a first (no boosting) mode, a second (single) boostedmode and third (double) boosted. For the example of FIG. 10 where thehigh-side and low-side voltage inputs are +VSUP and ground respectively,this means the first mode will have switching voltages of 0V and +VSUP,the second mode will have switching voltages of +VSUP and +2VSUP and thethird mode will have switching voltages of +2VSUP and +3VSUP.

In the first mode of operation, switching paths SW3A, SW3B, SW4A andSW4B may all be closed so as to charge both the first and secondcapacitors. This means that the input supplies are connected to theswitching voltage nodes and any load current can be supplied from theinput supplies. In an alternative state of operation, the switches SW3Aand SW3B could be opened so that the capacitors 1001 and 1002 providethe load current.

In operation in the second mode, the boosting could be provided by thefirst boost sub-stage, i.e. with switching path SW3C closed (and SW3Aand SW3B open) and the second boost sub-stage may be operated to passthrough the voltages without any boosting, i.e. with switching pathsSW4A and SW4B closed (and SW4C open). Alternatively, the first boostsub-stage may effectively be bypassed, with switching path SW3A closed(and SW3C and SW3B open) and the boosting provide by the second boostsub-stage with switching path SW4C closed (and SW4A and SW4B open).

The switching driver 401 illustrated in FIG. 10 is thus also operable inthree different driver operating modes, wherein the output range of theswitching driver in each mode is equal to the input voltage for theswitching driver 401 and the full output range of the driver is thusequal to three times the input voltage. The maximum voltage stressacross any of the switching paths is limited to magnitude of the inputvoltage and thus for an input voltage equal to 20V, a drain-sourcevoltage tolerance of 20V would be sufficient for the transistors of theswitching paths.

However, the peak high switching voltage for the switching driver ofFIG. 10 is, in this example, equal to +3VSUP compared to a maximumvoltage of +2VSUP in the example of FIG. 5. In some applications thismay require the transistor devices to have a sufficiently large drain tosubstrate breakdown voltage.

The switching driver illustrated in FIG. 10 may be implemented by NFETdevices for each of the switching paths. FIG. 11 illustratesschematically one example of suitable NFET devices 1101-1 and 1101-2.The devices are formed on a substrate 1102 and comprise source and drainregions 1103 and 1104, bulk region 1105 and gate 1106. In this example,however, the source region 1103 is formed within the bulk region 1105within an N-well 1107, but the drain region 1104 is formed within theN-well.

An isolation region 1108 may isolate the devices from one another and,in use, the isolation region 1108 and substrate 1102 may be grounded. Inuse, the N-well 1107 may be driven to a voltage that is the same as thedrain voltage.

FIG. 10 illustrates an example, where there are two boosting sub-stagesfor providing selective positive boosting, but it will be understoodthat other arrangements are possible, for instance there may be moreselective boosting sub-stages to allow more output voltage ranges and atleast some of the of the multiple boosting sub-stages could providenegative boosting.

In general, therefore embodiments provide a switching driver forgenerating a drive signal for driving a load, where the switching driveris operable in a plurality of different modes, where each of the modesinvolves switching between different switching voltages so that theoutput range of the driver varies in the different modes, and where theoutput range for each mode corresponds to only a subset of the fulloutput range of the driver.

In some examples the switching driver may be arranged to drive atransducer load in a single-ended configuration. Referring back to FIG.4, the switching driver 401 may therefore be arranged to provide a firstdriving signal at the driver output node 404 for driving the load 104,where the other side of the load is held at a DC voltage. The DC voltagecould, for example, be a voltage which is at a midpoint of the fulloperating range of the switching driver. For instance, for the examplediscussed with reference to FIG. 5, where the switching driver isoperable between a peak high-side voltage of +2VSUP in the second modeand a peak low-side voltage of −VBST in the third mode, the midpointvoltage is +VSUP/2. In some examples the level of the DC voltage at theother side of the load may be varied in use, as will be described inmore detail below.

In some implementations, however, a driver circuit may comprise twoswitching drivers arranged to drive a load in a BTL configuration. FIG.12 illustrates a driving circuit 1200 according to an embodiment withrespective first and second switching drivers 401-1 and 401-2 fordriving the load in a BTL arrangement. Each of the switching drivers401-1 and 401-2 may be a switching driver according to any of theembodiments described herein. In the example of FIG. 12 the twoswitching drivers 401-1 and 401-2 are provided with the same high-sideand low-side voltage inputs as one another, in this example VBST andground.

FIG. 12 illustrates that the switching of each switching driver 402-1and 402-2 may be controlled by a controller 1201. The controller 1201receives the input signal Sin and based on the input signal Sin,determine the appropriate driver mode of operation for each of theswitching drivers 401-1 and 401-2 and generate switching control signalsfor controlling the relevant switches of the network of switches so asto set an appropriate switch state to select the mode of operation. Thecontroller also generate the relevant switching control signals for theoutput switch of the output stage of each of the switching drivers 401-1and 401-2 to alternate between the relevant switching voltages with anappropriate duty cycle so as to provide the desired voltage at eachdriver output node and hence the desired differential voltage across theload.

The controller 1201 thus controls the driver mode of operation of eachof the switching drivers 401-1 and 401-2 so as to provide a desiredoutput voltage across the load. The controller 1202 controls theindividual driver modes of the drivers 401-1 and 401-2 to provide anappropriate overall BTL operating mode of the driver circuit to providethe desired output signal across the load.

For instance, consider the example where each of the switching drivers401-1 and 401-2 is a switching driver as discussed with reference toFIG. 5, and thus is operable in a first driver mode with a driver outputvoltage in the range 0V to +VSUP, a second driver mode with a driveroutput voltage in the range +VSUP to +2VSUP or a third driver mode witha driver output voltage in the range −VSUP to 0V. The first mode can beseen as providing a mid-range drive voltage, with the second and thirdmodes provides high-range (more positive than the mid-range) andlow-range (more negative than the mid-range) driver voltagesrespectively. For the example of the switched driver of FIG. 5, thefirst mode can be seen as an unboosted mode of operation of the driveras the voltage supplies received by the switching driver are used as theswitching voltages. The second mode can be seen as a positive boostedmode and the third mode can be seen as a negative boosted mode.

To generate a differential voltage across the load with a magnitude inthe range of 0V to +VSUP, each of the drivers 401-1 and 401-2 may beoperated in the first driver mode, i.e. with a driver voltage rangebetween 0V and +VSUP. In effect the first switching driver 401-1generates a first drive signal in the range of 0V to +VSUP and thesecond switching driver 401-2 generates a second drive signal in therange of 0V to +VSUP.

Assuming that a positive voltage across the load corresponds to thevoltage at the output node of first switching driver 401-1 being morepositive than the voltage at the output node of the second switchingdriver 401-2, then to provide a positive voltage, the duty cycle for thefirst switching driver 401-1 (in terms of proportion of time spent atthe high-side switching voltage) should be greater than that for thesecond switching driver 401-2. The differential drive signal across theload may thus be varied between +VSUP (with the first drive signal at avoltage +VSUP and the second drive signal at a voltage of 0V) and −VSUP(with the first drive signal at a voltage +VSUP and the second drivesignal at a voltage of 0V)

In some applications the controller could control the two switchingdrivers 401-1 and 401-2 so that the drivers switch synchronously inantiphase, i.e. such that the switching drivers switch between theswitching voltages at the same time as one another so that the outputnode of the first driver 401-1 is connected to its respective high-sideswitching voltage whilst the output node of the second driver 401-2 isconnected to its respective low-side switching voltage and vice-versa.In some implementations, however, it may be beneficial, at least in someuse cases, for the switching of the two output stages of the switchingdrivers to be asynchronous, i.e. such that the switching drivers 401-1and 401-2 may switch between their respective switching voltages atdifferent times to one another such both switching drivers may beconnected to their respective high-side or low-side switching voltage atthe same time for at least part of the duty cycle.

To provide a differential voltage across the load of a greatermagnitude, the switching drivers on the opposite sides of the load maybe operated in different driver modes from one another. For instance, adifferential voltage across the load with a magnitude in the range ofVSUP to 3VSUP could be achieved by operating the switching driver on oneside of the load in the second driver mode, to provide a driver voltagein the range +VSUP to +2VSUP whilst the switching driver on the otherside of the load is operated in the third driver mode, to provide adriver voltage in the range 0V to −VSUP.

For instance, a positive differential voltage across the load could varyfrom +VSUP (with the voltage of the first driver signal from driver401-1 at a voltage equal to +VSUP and the voltage of the second driversignal from driver 401-2 at 0V), to +3VSUP (with the voltage of thefirst driver signal from driver 401-1 at a voltage equal to +2VSUP andthe voltage of the second driver signal from driver 401-2 equal to−VSUP).

For a relatively low magnitude differential voltage across the load,i.e. a differential signal in the range −VSUP to +VSUP, the controller1201 could control the switching drivers 401-1 and 401-2 to each operatein the first driver mode (i.e. each generating a mid-range voltage).This can be seen as a first, low signal level BTL mode of operation ofthe driver circuit. For higher magnitudes of differential voltage acrossthe load, i.e. a magnitude greater than VSUP (up to the maximummagnitude of 3VSUP), the controller 1201 could control one of thedrivers 401-1 and 401-2 to operate in the second mode of operation (toprovide a more positive drive voltage) and the other to operate in thethird mode of operation (with a more negative drive voltage). Operatingin this way, with one driver operating in the second mode and one driveroperating in the third mode can be seen as a high-signal level BTL modeof the driver circuit. Which of the drivers 401-1 and 401-2 operates inthe second mode and which in the third will be controlled depending uponwhether a positive or negative differential voltage is required acrossthe load.

In some implementations the controller 1201 could be implemented to swapbetween just these BTL modes of operation, i.e. to operate in thelow-signal BTL mode for differential signal magnitudes below VSUP andthen swap to the relevant positive or negative high-signal BTL mode asappropriate when the differential signal magnitude increases.

However, it can, in some implementations, be beneficial to also operatewith one of the switching drivers in the first driver mode of operation(to provide a mid-range drive voltage) whilst operating the otherswitching driver in one of the second or third modes of operation.

For example, the first switching driver 401-1 could be operated in thesecond driver mode to switch between voltages in the range of +VSUP to+2VSUP, whilst the switching driver 401-2 is operated in the first modeto switch between 0V and +VSUP. Thus, the first switching driver 401-1generates a first drive signal with a voltage in the range of +VSUP to+2VSUP whilst the second switching driver generates a second drivesignal in the range of 0V to +VSUP. This could allow a differentialvoltage in the range of 0V (with the first drive voltage from driver401-1 at +VSUP and the second drive voltage from driver 401-2 at +VSUP)to +2VSUP (with the first drive voltage from driver 401-1 at +2VSUP andthe second drive voltage from driver 401-2 at 0V).

Alternatively, the first switching driver 401-1 could be operated in thefirst driver mode, and switched between 0V and +VSUP, whilst the secondswitching driver 401-2 is operated in the third mode to switch between−VSUP and 0V. Thus, the first switching driver 401-1 generates a firstdrive signal with a voltage in the range of 0V to +VSUP whilst thesecond switching driver 401-2 generates a second drive signal with avoltage in the range −VSUP and 0V, to apply a differential signal acrossthe load in the range of +VBST to +2VBST. Again, this could allow adifferential voltage in the range of 0V (with the first drive voltagefrom driver 401-1 at 0V and the second drive voltage from driver 401-2at 0V) to +2VSUP (with the first drive voltage from driver 401-1 at+VSUP and the second drive voltage from driver 401-2 at −VSUP).

Operating the switching drivers 401-1 and 401-2 with one driveroperating in the first driver mode (to provide a mid-range voltage)whilst operating the other switching driver in one of the second orthird driver modes, can thus be seen as an intermediate signal-BTL modeof operation.

Table 1 below summarises one example of how different ranges ofdifferential output voltages Vdiff across the load may be generated byvoltages Vx at the outputs of the first and second switching drivers401-1 and 401-2 where the high-side voltage input, i.e. VSUP, is 20V andthe low-side voltage input is ground.

TABLE 1 Vdiff Vx 401-1 Vx 401-2 0-20 V 0 to 20 V 0 to 20 V BTLlow-signal mode (first driver mode) (first driver mode) 20-40 V BTLintermediate 20 to 40 V 0 to 20 V signal mode (second driver mode)(first driver mode) 40-60 V 20 to 40 V −20 to 0 V BTL high signal mode(first driver mode) (third driver mode)

Differential voltages of the opposite polarity across the load can beachieved in a similar manner by swapping the relevant driver modes ofoperation of the first and second switching drivers 401-1 and 401-2.

It will thus be clear that, in this example a differential drivingsignal can be applied across the load which can vary in the range from+3VSUP to −3VSUP, i.e. a peak-to-peak differential voltage range equalin magnitude to six times the input voltage for the switching drivers401-1 and 401-2. As discussed above, the switching drivers 401-1 and401-2 may be implemented so the maximum voltage stress across theswitching paths is equal to the input voltage, i.e. VSUP, and thus theswitching drivers may only require a voltage tolerance which is of theorder of ⅙^(th) of the peak-to-peak differential output of the drivercircuitry 1200. Thus, the driver circuitry 1200 may utilize transistors,for example MOS transistors, having less breakdown voltage than thepeak-to-peak differential output voltage, for example approximately ⅙ththe breakdown voltage of the peak-to-peak differential output voltage ofthe driver circuitry.

Table 1 illustrates just one example, however, and the controller maycontrol the respective driver modes of operation according to differentdifferential output ranges. The controller may operate the first andsecond switching drivers in a given differential or BTL mode ofoperation, where each differential or BTL mode of operation involves adifferent combination of individual driver modes of the first and secondswitching drivers.

Driver circuitry which thus drives a load with two switching drivers ina BTL configuration, where the switching voltages on both sides of theload can be separately varied represents another aspect of thisdisclosure. Thus, in general, at least some embodiments relate to adriver circuit for driving a transducer based on an input signalcomprising first and second switching drivers configured to drive thetransducer in a bridge-tied-load configuration; wherein each of thefirst and second switching drivers comprise an output stage forcontrollably switching a driver output node between first and secondswitching voltages with a controlled duty cycle. Each of the first andsecond switching drivers is operable in a plurality of driver modes,wherein the first and second switching voltages are different in each ofsaid modes. A controller controls the driver mode of operation and dutycycle for each of the first and second switching drivers, such that thefirst and second switching drivers may be operable in different drivermodes from one another with different switching voltages.

The controller 1201 may thus control the driver mode of operation ofeach of the switching drivers 401-1 and 401-2, and the respective dutycycle, so as to provide a desired differential drive voltage based onthe input signal Sin and may vary the driver mode of operation of theswitching drivers as the input signal varies.

Thus, for example, consider the operation illustrated in table 1 above,and consider that the required differential output voltage, based on theinput signal is 0V. The controller 1201 may operate both switchingdrivers in the first driver mode (0 to 20V), i.e. operate in thelow-signal level BTL mode, and may control the switch drivers with equalduty cycles. If the value of the input signal then rises over time, thecontroller could increase the duty cycle of the first switchingdriver/reduce the duty cycle of the second switching driver to providethe increased differential output voltage. If the required voltage forthe drive signal continues to rise, in particular to a voltage ofgreater than 20V, the controller could then swap the mode of operationof the first switching driver to the second driver mode of operation (20to 40V) with an appropriate duty cycle.

In theory, during the operation in the first driver mode, the duty cycleof the first switching driver 401-1 could be increased to 100% whilstthe duty cycle of the second switching driver 401-1 was decreased to 0%to provide a differential output voltage of 20V. In practice, however,limits on the clock speeds and the speed of the response of thetransistors means that duty cycles of 100% and 0% may not be practicallyrealisable. Thus, in some implementations it may not be practical togenerate a differential output voltage of 20V in the low signal levelBTL mode, with the first and second switching drivers 401-1 and 401-2each operating in the first driver mode with switching voltages of 0V to20V.

However, a differential voltage of 20V could be achieved by operating inthe intermediate signal level BTL mode, with one switching driveroperating in the first driver mode and the other operating in one of thesecond or third driver modes. Thus, the first switching driver could beoperated in the second driver mode (in this example with switchingvoltages of 20V and 40V) to generate a first drive signal with a voltagegreater than 20V, say 22V for example, whilst the second switchingdriver 401-2 is operated in the first mode (with switching voltages of0V and 20V) to provide a voltage greater than 0V, say 2V, so that thedifferential voltage between the first and second driving signalscorresponds to the required differential output. This allows the desiredoutput signal to be generated whilst operating both switching driverswith valid duty cycles.

In some cases, the duty cycle for a switching driver could be validlyvaried within certain limits, e.g. between lower and upper or minimumand maximum duty cycle limits, say between 5% and 95% or between 10% and90% in some implementations, although different may be used in otherimplementations (which may be, but need not be, symmetrical about 50%).In use, the controller 1201 may operate each of the switching drivers401-1 and 401-2 in a given mode of operation and controllably vary theduty cycle with input signal between these limits, but once the relevantlimit for at least one of the switching drivers is reached thecontroller 1201 may swap the driver mode of operation of one of theswitching drivers at the duty cycle limit, and controllably vary theduty cycles of both drivers, based on the new operating mode, to providean equivalent differential voltage. This can, however, vary thecommon-mode voltage of the first and second drive signals applied to theload in the different BTL modes of operation.

Table 2 below illustrates one example of how various differentdifferential voltages could be achieved, for the example where switchingdrivers are operable in a first driver mode with switching voltages of0V and 20V, a second driver mode with switching voltages of 20V and 40Vand a third driver mode with switching voltages of 0V and −20V. Table 2illustrates the differential output voltage Vdiff, and for each of thefirst and second switching drivers 401-1 and 401-2, the respective drivevoltage Vx (i.e. the average voltage at the driver output over the dutycycle), the switching voltages SW and the duty cycle DC (in terms ofproportion of time spent at the high-side switching voltage). Table 2also illustrates the resulting common-mode voltage VCM of the first andsecond driver signals. Note that the reference to the common-modevoltage or common-mode voltage component refers to the averagecommon-mode voltage component of the drive signals, i.e. determined overa full switching cycle. It will be understood that during the cycle,there may be some common-mode ripple depending on the timing ofswitching between the switching voltages on either side of the load.

TABLE 2 First Driver 401-1 Second driver 401-2 Vdiff Vx1 SVs DC Vx2 SVsDC VCM 12 16 20/0  80% 4 20/0  20% 10 14 17 20/0  85% 3 20/0  15% 10 1618 20/0  90% 2 20/0  10% 10 16 22 40/20 10% 6 20/0  30% 14 18 23 40/2015% 5 20/0  25% 14 20 24 40/20 20% 4 20/0  20% 14 22 25 40/20 25% 320/0  15% 14 24 26 40/20 30% 2 20/0  10% 14 24 22 40/20 10% −2    0/−2090% 10

In the example of FIG. 2, the duty cycle of each of the switchingdrivers may be controllably varied with minimum and maximum duty cyclelimits of 10% and 90%. It can be seen that, in this example, adifferential voltage of up to 16V can be generated by operating in thelow-signal level BTL mode, i.e. with both switching drivers operating inthe first driver mode with valid duty cycles in the range of 10% to 90%.In this example the duty cycles of the switching drivers, in thelow-signal level BTL mode, are controlled to vary equally and oppositelyabout a duty cycle of 50% and thus the common mode voltage is equal to10V.

At the differential output of 16V the duty cycle of the first switchingdriver 401-1 is at the maximum duty cycle limit of 90% and cannot beincreased further, and likewise the second switching driver 401-2 is atthe minimum duty cycle limit of 10%. When the duty cycle limit isreached, the controller transitions the first switching driver tooperate in the second driver mode, with switching voltages of 20V and40V, and sets the duty cycle for the first switching driver to theminimum duty cycle limit. At the minimum valid duty cycle, in thisexample 10%, the voltage of the first driver signal is thus 22V. Toprovide the correct differential voltage of 16V, the operation of thesecond switching driver is maintained in the first driver mode, but theduty-cycle carried to provide a second drive signal with a voltage of6V, which, in this example, involves operating with a duty cycle of 30%.The controller thus effectively transitions from the low signal levelBTL mode of operation to the intermediate signal level BTL mode ofoperation by transitioning the driver mode of operation on one side ofthe load (whilst maintaining the driver mode on the other side of theload) and maintains the same differential voltage across the load whentransitioning. This does however result in the common-mode component ofthe first and second drive signals increasing to 14V.

If the required differential output Vdiff increases further, the dutycycle of the first switching driver 401-1 may be increased, with theduty cycle of the second switching driver 401-2 being correspondinglyreduced, i.e. the duty cycles may be varied equally and oppositely,which maintains the common-mode voltage at a substantially constantvalue (14V in this example) when operating in the intermediate signallevel BTL mode. The duty cycles may be varied until the duty cycle ofthe second switching driver 401-2 reaches the minimum limit of 10%,which in this example occurs for the differential output voltage of 24V.

At this point, the mode of operation of the second switching driver401-2 may be changed to the third driver mode, with switching voltagesof −20V and 0V, and the duty cycle of second switching driver set to themaximum duty cycle limit, which in this example is 90% which provides adriver voltage Vx from the second driver of −2V. To provide the desireddifferential voltage of 24V, first switching driver 401-1 may continueto operate in the second mode and the duty cycle controlled to provide avoltage for the first drive signal of 22V, which in this casecorresponds to the minimum duty cycle of 10%. Thus the controller 1201effectively transitions to the high-signal level BTL mode of operation.This provides the desired differential output and also returns thecommon-mode voltage to 10V.

Further increases in the differential output voltage Vout can then beachieved by continuing to operate in the high-signal level BTL mode,with the first switching driver 401-1 in the second driver mode and thesecond switching driver 401-2 in the third driver mode, and increasingthe duty cycle of the first switching driver whilst decreasing the dutycycle of the second switching driver. Applying equal and oppositechanges to the duty cycle will maintain the common-mode voltage at 10V.

It should be understood that the duty cycle limits of 10% and 90% arejust an example and other limits could be implemented.

Operating in this way, with a variation or jump of the common-modevoltage, which may be referred to as operating with a bumpedcommon-mode, thus allows voltages around 20V (in this example) to beachieved with valid duty cycles.

FIG. 13 illustrates some example voltage waveforms to illustrate thisprinciple. FIG. 13 illustrates, in the second and third waveforms fromthe top, the voltages Vx1 and Vx2 of the drive signal at the respectivedriver output nodes (in terms of the average voltage over the course ofa duty cycle). FIG. 13 also illustrates, in the top waveform, theresultant differential voltage Vdiff across the load. The lower twowaveforms illustrate the switching waveforms at the respective driveroutput node, for parts of the waveforms illustrated in the top plots,and illustrate how the switching voltages and duty cycles may varyacross a mode transition.

In this example, each of the switching drivers 401-1 and 401-2 isoperable in three modes, similar to those discussed above with referenceto FIG. 3. FIG. 13 illustrates the example where the differential outputvoltage, which depends on the relevant input signal, varies positivelyand then negatively with a relatively high amplitude.

FIG. 13 illustrates that initially the required differential voltageacross the load is zero, and both the switching drivers operate toprovide the same quiescent voltage level, which in this case is amidpoint voltage between V1 and V2. Both switching drivers 401-1 and401-2 may thus operate in the first mode switching between switchingvoltages V1 and V2, i.e. the low-signal BTL mode, and both switchingdrivers may each initially operate with a duty cycle of 50%. As therequired differential voltage increases, the duty cycle of the firstswitching driver 401-1 is increased, with an equal and opposite changeto the duty cycle of the second switching driver 401-2, so the drivervoltages Vx1 and Vx2 vary equally and oppositely.

As the required differential voltage Vdiff increases, the firstswitching driver 401-1 transitions to operating in the second mode, withswitching voltages of VH and V1. As discussed above, however, it may notpractically be possible for the switching driver to operate with a dutycycle that goes as high as 100% or a low as 0%. Thus the duty-cycle ofthe first switching driver will go from a duty cycle less than 100% inthe first mode to a duty-cycle greater than 0% in the second mode. Assuch the average of the drive voltage Vx1 (over the course of theswitching cycle) jumps from a value below V1 to a value above V1. Thesame jump in voltage is thus applied to the driver voltage Vx2 of thesecond switching driver 401-2 to maintain the correct differentialvoltage. The second switching driver 401-2 thus continues to operate inthe first mode and the system operates in the intermediate signal levelBTL mode.

As the required differential voltage Vdiff increases further, the secondswitching driver transitions to the third mode, with switching voltagesof V2 and VL, to provide the high-signal level BTL mode. Again, thepractical duty cycle limits result in a jump in the voltage Vx2 acrossthe transition, and thus a corresponding voltage jump is applied to Vx1to maintain the correct differential voltage Vdiff.

FIG. 13 also illustrates that similar jumps may apply when transitioningback to the intermediate signal level BTL mode and then the low-signallevel BTL mode and also for the negative part of the cycle.

FIG. 13 also illustrates examples of the switching waveforms across twoof the mode transitions. In the first case, both switching drivers areinitially operating in the first mode with switching voltages V1 and V2,and then at a time illustrated by arrow 1301, the second switchingdriver transitions to the first mode, with switching voltage VH and V1,i.e. to the BTL intermediate signal level mode for a negativedifferential voltage. Just before the transition the output node of thesecond switching driver 401-2 is switching between the switch voltage V1and V2 with a duty cycle at a defined duty cycle maximum. After thetransition, illustrated by arrow 1301, the output node of the secondswitching driver switches between the voltages VH and V1, with a minimumduty cycle. The output node of the first switching driver 401-1continues to switch between the voltages V1 and V2, but the duty cycleis increased across the transition 1301.

The second case illustrates the transition to the high-signal level BTLmode of operation, where the first switching driver transitions 1302 tothe third driver mode with switching voltage V2 and VL and an increaseof duty cycle from minimum to maximum and the second switching drivercontinues to switch between the voltages VH and V1 but with a reduceduty cycle.

FIG. 14 illustrates how the duty cycle of the switching drivers may varyon each side of the load to provide a differential output waveform suchas illustrated in the top plot FIG. 13, in this example where themaximum and minimum duty cycles are 75% and 25% respectively.

It will also be noted that, as in these example, where the highswitching voltage in the first driver mode is the low switching voltagein the second driver mode, and the difference between the switchingvoltages are the same in each of the modes, then changing from the firstdriver mode to the second driver mode can be considered mathematicallythe same as adding an additional 100% to the duty cycle in the firstmode. In other words a change from 90% duty cycle in the first drivermode to 10% duty cycle in the second driver mode, can be considered tomathematically the same as an increase from 90% to 110% for theswitching voltages of the first driver mode. This change can thus beseen as increase in duty cycle of 20%, and thus to maintain the samedifferential voltage on such a mode transition, an equivalent change induty cycle is made to the other driver, i.e. an increase in duty cyclefrom 10% to 30%. In other words, the magnitude of a change in duty cycleapplied to both drivers equals 100%, that is if the duty cycle of thefirst driver is reduced from 90% to 10% on a mode change, whichrepresents a change in duty cycle of 80%, the corresponding change induty cycle applied to the second driver is 20%.

It should also be noted that defined maximum and minimum limits for thepurposes of a mode a mode transition need not be the same as apractically achievable duty cycle limit. For instance, it may in somecases be advantageous to swap between a maximum duty cycle of 75% and aminimum duty cycle of 25%. This particular combination means that thechange in duty cycle on each side of the load over a transition is 50%and on each transition the duty cycle on one side of the loadtransitions from 75% to 25% and the duty cycle in the other side of theload transitions from 25% to 75%. This can be advantageous in terms ofminimising discontinuities on mode transition.

The controller can therefore be seen as implementing three BTL modes ofoperation, (i) a low-signal level BTL mode, with both drivers operatingin the first driver mode to provide a mid-range voltage; (ii) anintermediate signal level BTL mode with one driver operating in thefirst driver mode and the other driver operating in the second or thirddriver mode to provide a high (relatively positive) or low (relativelynegative) range voltage; and (iii) a high-signal level BTL mode with onedriver operating in the second driver mode to provide a high (positive)voltage range and the other driver operating in third driver mode toprovide a low (negative) voltage range. The controller may operate in agiven mode and controllably vary the duty cycle of both drivers withindefined maximum and minimum limits of duty cycle. The controller maycontrol the duty cycles to vary with input signal level within thelimits, so that a common-mode component does not substantially vary withsignal level in a given operating mode. If required, the controller maytransition between modes, where each transition involves changing thedriver mode, i.e. the switching voltages, on one side of the load only.This, if the input signal changes increase say the controller maytransition from the low-signal level BTL mode to the intermediate signallevel mode by changing the switching voltages one side of the load andthen, if the input signal continues to increase, later transition to thehigh-signal level mode by changing the driver mode on the other side ofthe load.

The controller may be configured so that the common-mode component ofthe first and second driver signals is substantially the same in thelow-signal level BTL mode and the high-signal level BTL mode, which maycorrespond to a midpoint voltage between the switching voltages used inthe first driver mode. The common-mode component may be different whenoperating in the intermediate signal level BTL mode.

In general therefore, the controller may be configured to implement atransition in mode of one of the switching drivers, from the presentdriver mode to a new driver mode, by initially controlling the switchingdriver (undergoing the mode change) to operate in the current drivermode with a first duty cycle, whilst controlling the other switchingdriver with a second duty cycle. The controller may then transition therelevant switching to operate in the new driver mode (with differentswitching voltages) with a modified duty cycle and may also modify theduty of the other switching driver to maintain the same differentialvoltage across the transducer. The controller may, in particular, beconfigured to transition between different driver modes of operationwhen the duty cycle of at least one of the first and second switchingdrivers reaches a maximum or minimum duty cycle limit. The controllermay implement the transition by changing the driver mode of one of thefirst and second switching drivers at said maximum or minimum limit ofduty cycle and vary the duty cycle of that one of the first and secondswitching drivers to the other limit of duty cycle (i.e. from maximum tominimum of vice versa) whilst maintaining the driver mode of the otherof the first and second switching drivers and applying a variation induty cycle to maintain a magnitude of a differential component of thefirst and second driver signal.

Note, similar principles could be applied to a driving circuit fordriving a load in a single ended configuration, if the DC voltage on theother side of the load could be controllably varied, so to avoiddiscontinuous in the drive signal applied to the load due to switchingdriver being unable to achieve a duty cycle (i.e. modulation index) of100% or 0%.

For example, consider that a switching driver is configured to drive oneside of a load in a single ended configuration and is operable in afirst mode with switching voltages 0V and 20V, and a second mode withswitching voltages of 20V and 40V. Whilst operating in the first mode,the voltage on the other side of the load may be held at a first DCvalue, say 2V purely as an example. The duty cycle of the switchingdriver may be controllably varied within a range up to a defined limit,which could, for example, be a duty cycle or modulation index of 90%,which could lead to a voltage for the drive signal from the switchingdriver of 18V, leading a voltage of 16V across the load. To transitionbetween modes of operation, to allow for higher voltages to be applied,the switching driver may initially be operated in the first mode at aduty cycle of 90%, and then switched to operate in the second mode witha duty cycle of 10%. At the same time, the DC voltage on the other sideof the load may be increased to 6V. FIG. 15 illustrates the relevantswitching waveform 1301 for the output node of the switching driver andthe voltage 1302 on the other side of the load for such a transition andthe load current. It will be seen that by changing the duty cycle ormodulation index from 90% to 10% and also changing the DC voltage, thevolt-second balance is maintained as well as the differential voltage.

In a bridge-tied-load configuration, where both sides of the load aredriven by a switching driver, second order effects can appear. Theinductor current matches the output-based duty cycle and discontinuityof charge transfer can occur.

FIG. 16 illustrates some example waveforms for driver circuitrycomprising first and second switching drivers according to embodimentsof the disclosure and illustrates a mode transition for the switchingvoltages for the first switching driver. FIG. 16 illustrates two sets ofwaveforms, (a) and (b) in each case showing the differential outputvoltage Vdiff, the first and second output currents 1601 and 1602 forthe first and second switching drivers respectively and the voltages1603 and 1604 at the output nodes for the first and second switchingdrivers respectively during a mode transition.

In the top set of waveforms (a), it can be seen that the first switchingdriver is initially operating with the same switching voltages as thesecond switching driver. It can be seen that the duty cycle for thefirst switching driver is relatively high, i.e. the relevant drivevoltage is near the maximum for the driver mode of operation. The drivermode of operation for the first switching driver then changes, and itstarts switching between the previous high voltage and a higher boostedvoltage, but now with a relatively short duty cycle. It can be seenthat, in this example, the change to the switching voltages andresultant change in duty cycle results in a disturbance in the currentwaveforms, with a consequent disturbance in the output voltage.

In some implementations this can be mitigated by a controlled variationin the switching timing. The lower set of waveforms (b) show a similartransition, but the timing of switching to the high-side voltage in thesecond mode is controlled to occur at a suitable point which does notresult in any significant disturbance in the current waveforms.

At the point of a mode transition the switching voltages on one side ofthe load vary and there is a change in duty cycle on both sides of theload. This can result in the pattern of load current flow changing asdiscussed with reference to FIG. 16. In particular there may be areversal of load current ramping at the point of switching which resultsin a discontinuity in load current. The mode transitions may thereforebe implemented to avoid an unwanted discontinuity in load current. Thismay, in some examples, be achieved by applying a phase shift to a PWMcarrier waveform used to generate the PWM control signals for the outputstages of the switching drivers. The phase shift may be based on theamount of change in duty cycle.

Generally the duty cycles for the drivers on both sides of the load maybe generated by a modulator by comparing a modulator input with acarrier waveform, which will generally be a sawtooth or triangularwaveform. The input to the modulator is based on, or derived from theinput signal, possibly with some adjustment to reflect the operatingmode of the switching drivers—and generally the modulator input fordriving one side of the load is an inverted version of the modulatorinput for driving the other side of the load. The modulator compares therelevant input to the carrier waveform and generates the PWM controlsignal.

FIG. 17 illustrates two sets of example waveforms and how the timing maybe controlled on a mode transition. FIG. 17 illustrates a carrierwaveform, in this example a sawtooth waveform that ramps up and downover the course of a switching cycle. FIG. 17 also illustrates thevoltages Vx1 at the output of the first drivers and Vx2 at the output ofthe second driver and also the difference Vx1−Vx2 and the resulting loadcurrent from the first switching driver.

FIG. 17 illustrates four switching cycles and illustrates an examplewhere, for the first two cycles, both of the first and second switchingdrivers are operating in the first driver mode. In this example, bothdrivers output the relevant high switching voltage at the start of aswitching cycle and then switch to the low switching voltage when thecarrier waveform reaches the value of the relevant input. In theillustrated example the first driver has a relatively high duty cycleand the second driver has a relatively low duty cycle. This results inperiods of high and low differential voltage across the load and thecurrent IR from the first driver ramping up and down accordingly, withthe relevant periods depending on the respective duty cycles. It will benoted that in this example the load current ramps down at the end of oneswitching period and start of another, when both drivers are outputtingthe same high switching voltage.

The top set of waveforms illustrates that the first switching drivertransitions to the second driver mode of operation at the end of thesecond switching cycle. Now, at the start of the switching cycle, thefirst driver is outputting the high switching voltage for the seconddriver mode, which is higher than high switching voltage that is outputby the second driver. This results in a relatively high differentialvoltage across the load and results in change in the load current slopewhich results in a discontinuity. As a result of the change in mode, thefirst driver now operates with a relatively low duty cycle and the dutycycle of the second driver is increased to maintain the correctdifferential voltage.

The lower set of waveforms illustrates similar operation for the firsttwo switching cycles, but in this case at the point of mode transitionthere is a phase shift to the PWM carrier. In particular the carrier isphase shifted to an extent such that the output of first switchingdriver is at the new low switching voltage. This keeps the voltage Vx1of the first switching driver at the same level across the modetransition and avoids a change in slope of load current. The voltage Vx2of the second switching driver will then later switch to the lowswitching voltage when the carrier reaches the appropriate level. For asawtooth waveform, the phase shift applied (in terms of percent of thecycle period) may correspond to the shift in duty cycle divided by two,i.e. if the change in duty cycle were say 40% the phase shift maycorrespond to a time shift of 20% of the switching period. Thismaintains the same load current profile during the mode transition andavoid discontinuity.

In embodiments of the disclosure, the controller may thus be configuredto control the first and second switching drivers such that there is nosubstantial change in slope of load current from a driver output node atthe point of a mode transition. The controller may be configured, on achange in driver mode, to vary an amount of a mode correction applied tothe modulator input by an amount related to the magnitude of the voltagerange of the new operating mode. The controller may be configured toapply a phase shift to a PWM carrier waveform on a mode transition. Thephase shift applied may be based on the amount of change in duty cycle(applied to the switching driver which is not changing driver mode).When the carrier waveform is a sawtooth waveform, that ramps from aminimum value to a maximum value and back to the minimum value over thecourse of a cycle period, the controller may be configured such that thephase shift applied to the carrier waveform is proportion of the cycleperiod that corresponds to half the change in duty cycle for the secondswitching driver.

In some implementations BD-modulation techniques can be used to managethe discontinuity by switching between points of equivalent ripple. Thatis, the switching of the first and second switching drivers may beasynchronous and the output of both of the switching drivers may beconnected to the relevant high-side voltage or low-side voltagesimultaneously for at least part of the switching cycle.

The amount of ripple in the output from a switching driver will varyover time depending on the duty cycle of switched mode driver and thepeak-to-peak ripple will thus vary with duty cycle of the switchingdriver.

FIG. 18 illustrates one example of how the peak-to-peak ripple may varyover the range of duty cycles from 0 (i.e. 0%) to 1 (i.e 100%). Toreduce the extent of any discontinuity as a result of a mode transition,the transition may be implemented so that the duty cycles of theswitching drivers vary but maintain a substantially equal amount ofripple.

This could be implemented in different ways. In a first approach, theduty cycle of a switching driver could be changed from a first dutycycle D1 to a second duty cycle D2, where the duty cycles D1 and D2exhibit substantially the same peak-to-peak ripple as one another. Thisis illustrated by transition 1801 in FIG. 18 which indicates that theduty cycle of the switching driver could be varied from a first dutycycle D1 to a second duty cycle D2, the first and second duty cyclesbeing selected to provide equal amounts of ripple.

In a second approach, the duty cycles of both switching drivers could bechanged so that the amount of ripple seen differentially across the loadstays constant. For instance, as illustrated by transition 1802 in FIG.15, the duty cycle of the first switching driver could be varied from afirst duty cycle D3 to a second duty cycle D4, say from 25% to 75%,whilst the duty cycle of the second switching driver is changed from D4to D3, e.g. from 75% to 25%. In general the duty cycles of the first andsecond switching drivers may thus be varied so that an amount of rippledue to the duty cycle of the first switching driver before thetransition is the same as an amount of ripple due to the duty cycle ofthe second switching driver after the transition and vice versa.

In general, therefore, embodiments of the present disclosure relate toswitching drivers which are suitable for driving an output transducer,that are operable to provide a drive signal with an average voltagewithin a defined output voltage range, e.g. between a low voltage VL anda high voltage VH. The switching driver is operable in a plurality ofdifferent driver modes, wherein in each of the modes a driver outputnode is switched between two switching voltages with a controlled dutycycle, wherein the switching voltages are different for each mode andthe switching voltages in each mode provide only part, i.e. a subset, ofthe defined output voltage range.

The different switching voltages in the different driver modes maydefine non-overlapping voltage ranges, however this need not be thecase, and, in some embodiments, there could be some overlap between thevoltage ranges. For instance, the switching voltages in a first drivermode could be V1 and V2 and in a second driver mode one of the switchingvoltages may be a voltage which is between V1 and V2. For example theswitching voltages may be 0V and 20V in a first driver mode and say 16Vand 26V in a second driver mode.

In at least some embodiments the switching driver may comprise at leastvariable boost stage having first and second inputs for receivinghigh-side and low-side input voltages and comprising a network ofswitching paths and connections for a capacitor. The variable booststage may thus be a switched capacitor stage.

The switched capacitor converter stage may implement a boost function asneeded. When used for driving reactive loads, such as a piezotransducer, a relatively large capacitor can provide the charge neededwith one charge pumping cycle.

The variable boost stage controls the switching voltages supplied to aswitching output stage. The output stage, for instance, be controlled asa Pulse-Width Modulation (PWM) output stage. In some implementations thePWM stage may drive the load in series with an inductor, which allowslossless movement of charge between load and capacitor of the switchedcapacitor variable boost stage for charge recovery.

A driver circuitry according to at least some embodiments may thus beseen as a hybrid, e.g. capacitive and inductive, driver circuit.

The driver circuitry is configured to have floating supply domains, i.e.variable switching voltages in different operating modes, which allowsfor the switches of the driver circuit to have a relatively low Vdsmax(drain-source voltage tolerance). The driver circuit may be configuredsuch that the MOSFET well-to-substrate breakdown voltages aresufficiently high for the peak output voltages generated in use.

A first boost converter may provide boosting from a supply voltage, suchas a battery voltage to an intermediate supply VBST for input to thevariable boost stage.

At least some of the embodiments described above relate to a hybrid(e.g. capacitive and inductive) driver for a reactive (e.g.piezoelectric) load, where driver comprises: a charge pump, a PWMdriver, and a control system arranged to maintain linearity whiletransitioning the charge pump states.

The driver may be operable to produce N times the peak-to-peak outputvoltage, which can be N times the Vds breakdown voltage of the devicesby driving the deep N-wells in a signal dependent manner (where N=6, forexample).

At least some embodiments relate to a hybrid driver for a reactive loadsuch as a piezoelectric transducer, where the driver is arranged toreceive an input signal and to generate an output signal for driving thereactive load, the hybrid driver comprising: a driver stage, which may,for example, be a Pulse-Width-Modulation (PWM) driver, the driver stagearranged to receive the input signal and to generate the output signalfor driving a reactive load. The driver may also comprise a variableboost stage, such as a charge pump stage, arranged to generate a voltagesupply range for the driver stage based on a supply voltage, and thecharge pump stage may be controlled such that the voltage supply rangeis based on the range of the received input signal.

The driver may further comprise a first define boost stage or powerconverter stage, e.g. a DC/DC Boost Converter, the power converter stagearranged to receive a supply voltage and to generate a boosted supplyvoltage, wherein the charge pump stage generates the voltage supplyrange based on the boosted supply voltage. The supply voltage may befrom a battery or other power source.

The charge pump stage may comprise at least one capacitor, wherein thecapacitor size is selected based on the reactive load to be driven, suchthat a single charging cycle of the capacitor provides sufficient chargefor the driving of the reactive load.

Preferably, the driver stage drives an inductor arranged in series withthe reactive load, wherein the inductor allows lossless movement ofcharge between the load and a supply capacitor for charge recovery.

The hybrid driver may be implemented in a single-ended configuration.The hybrid driver may alternatively be implemented in a bridge-tied-load(BTL) configuration, to drive opposite terminals of the reactive load.For a BTL configuration, it will be understood that the hybrid drivercomprises complimentary charge pump and driver stages for either side ofthe load.

In at least implementations, the hybrid driver may comprise switchingdevices, preferably MOSFETs. In one example, N-type MOSFETs are used asthe switching devices. In an alternative example, a mixture of N-typeand P-type MOSFETs are used.

Preferably, the switching devices are at least partly controlled basedon the signal range of the output signal. In some implementations, wellsof the switching devices, e.g. deep N-wells, are driven based on thesignal range of the output signal. By driving the deep N-wells of theMOSFETs, it is possible to avoid P-well to deep N-well breakdown, orforward biasing of the deep N-well to the substrate or P-well of thedevices. Accordingly, the peak-to-peak output voltage of the hybriddriver can be a multiple of the Vds breakdown voltage of the devicesused.

The charge pump stage may be arranged to select a voltage supply rangeto be generated from a plurality of supply ranges. In some example, thecharge pump stage is arranged to select between 0-20V, 20-40V, or −20-0Vvoltage ranges.

The hybrid driver may be controlled to apply a common mode jump tolinearize the range transition. When driving the load differentially,the hybrid driver controls both the differential and the common modevoltage in order to keep both of the differential outputs in validoperating ranges and thus avoid distortion and artefacts during rangetransitions of the charge pump.

In some implementations, for an asymmetric differential configurationconsisting of one hybrid driver and a DC source, the driver iscontrolled to vary the modulation index of the driver while adjusting aDC source applied to the opposite side of the reactive load to eliminatediscontinuities between charge transfer phases of the charge pump stage.

For a BTL configuration, the charge pump stages of the driver may becontrolled to eliminate discontinuities between charge transfer phasesof the charge pump stages, for example by applying a BD-modulation tothe switching of the charge pump stages.

There is further provided a control method for a driver, preferably fordriving a reactive load such as a piezoelectric transducer, the drivercomprising a plurality of MOSFET switching devices, the methodcomprising the steps of: receiving an input signal, and generating anoutput signal based on the received input signal, the output signal atleast partly generated by switching of the MOSFET switching devices,wherein the method comprises the further step of: driving the wells ofthe MOSFET switching devices based on the signal range of the outputsignal, to prevent breakdown or forward biasing of the MOSFET switchingdevices.

The driver may be a hybrid driver comprising inductive and reactiveelements.

The driver may comprise a set of high-voltage switches, and the step ofdriving the wells may comprise operating the high-voltage switchessynchronously to the charge-pump during large signals.

There is also provided a system for driving a reactive load such as apiezoelectric transducer, the system comprising: a driver stage toreceive an input signal and to generate an output signal for driving areactive load, a charge pump stage arranged to generate a voltage supplyrange for the driver stage based on a supply voltage, and an inductorelement to be arranged in series with the reactive load such that thedriver stage drives the inductor element, wherein the inductor elementallows lossless movement of charge between the load and a supplycapacitor for charge recovery.

It will be understood that the driver stage and charge pump stage may beprovided by a hybrid driver as described above

Embodiments also relate to driver circuitry comprising two switchingdrivers configured to provide output drive signals for driving abridge-tied-load.

As mentioned, the switching driver may be suitable for driving an outputtransducer. The output transducer may be, in some implementations, be anaudio output transducer such as a loudspeaker or the like. The outputtransducer may be a haptic output transducer. In some implementation theoutput transducer may be driven in series with an inductor, i.e. theremay be an inductor in an output path between an output node of theswitching driver and the load. In some implementations the transducermay be a piezoelectric or ceramic transducer.

Embodiments may be implemented as an integrated circuit. Embodiments maybe implemented in a host device, especially a portable and/or batterypowered host device such as a mobile computing device for example alaptop, notebook or tablet computer, or a mobile communication devicesuch as a mobile telephone, for example a smartphone. The device couldbe a wearable device such as a smartwatch. The host device could be agames console, a remote control device, a home automation controller ora domestic appliance, a toy, a machine such as a robot, an audio player,a video player. It will be understood that embodiments may beimplemented as part of a system provided in a home appliance or in avehicle or interactive display. There is further provided a host deviceincorporating the above-described embodiments.

The skilled person will recognise that some aspects of theabove-described apparatus and methods, for instance aspects ofcontrolling the switching control signals to implement the differentmodes, may be embodied as processor control code, for example on anon-volatile carrier medium such as a disk, CD- or DVD-ROM, programmedmemory such as read only memory (Firmware), or on a data carrier such asan optical or electrical signal carrier. For some applications,embodiments may be implemented on a DSP (Digital Signal Processor), ASIC(Application Specific Integrated Circuit) or FPGA (Field ProgrammableGate Array). Thus, the code may comprise conventional program code ormicrocode or, for example code for setting up or controlling an ASIC orFPGA. The code may also comprise code for dynamically configuringre-configurable apparatus such as re-programmable logic gate arrays.Similarly, the code may comprise code for a hardware descriptionlanguage such as Verilog™ or VHDL (Very high-speed integrated circuitHardware Description Language). As the skilled person will appreciate,the code may be distributed between a plurality of coupled components incommunication with one another. Where appropriate, the embodiments mayalso be implemented using code running on a field-(re)programmableanalogue array or similar device in order to configure analoguehardware.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. The word “comprising” does not excludethe presence of elements or steps other than those listed in a claim,“a” or “an” does not exclude a plurality, and a single feature or otherunit may fulfil the functions of several units recited in the claims.Any reference numerals or labels in the claims shall not be construed soas to limit their scope

As used herein, when two or more elements are referred to as “coupled”to one another, such term indicates that such two or more elements arein electronic communication or mechanical communication, as applicable,whether connected indirectly or directly, with or without interveningelements.

This disclosure encompasses all changes, substitutions, variations,alterations, and modifications to the example embodiments herein that aperson having ordinary skill in the art would comprehend. Similarly,where appropriate, the appended claims encompass all changes,substitutions, variations, alterations, and modifications to the exampleembodiments herein that a person having ordinary skill in the art wouldcomprehend. Moreover, reference in the appended claims to an apparatusor system or a component of an apparatus or system being adapted to,arranged to, capable of, configured to, enabled to, operable to, oroperative to perform a particular function encompasses that apparatus,system, or component, whether or not it or that particular function isactivated, turned on, or unlocked, as long as that apparatus, system, orcomponent is so adapted, arranged, capable, configured, enabled,operable, or operative. Accordingly, modifications, additions, oromissions may be made to the systems, apparatuses, and methods describedherein without departing from the scope of the disclosure. For example,the components of the systems and apparatuses may be integrated orseparated. Moreover, the operations of the systems and apparatusesdisclosed herein may be performed by more, fewer, or other componentsand the methods described may include more, fewer, or other steps.Additionally, steps may be performed in any suitable order. As used inthis document, “each” refers to each member of a set or each member of asubset of a set.

Although exemplary embodiments are illustrated in the figures anddescribed below, the principles of the present disclosure may beimplemented using any number of techniques, whether currently known ornot. The present disclosure should in no way be limited to the exemplaryimplementations and techniques illustrated in the drawings and describedabove.

Unless otherwise specifically noted, articles depicted in the drawingsare not necessarily drawn to scale.

All examples and conditional language recited herein are intended forpedagogical objects to aid the reader in understanding the disclosureand the concepts contributed by the inventor to furthering the art, andare construed as being without limitation to such specifically recitedexamples and conditions. Although embodiments of the present disclosurehave been described in detail, it should be understood that variouschanges, substitutions, and alterations could be made hereto withoutdeparting from the spirit and scope of the disclosure.

Although specific advantages have been enumerated above, variousembodiments may include some, none, or all of the enumerated advantages.Additionally, other technical advantages may become readily apparent toone of ordinary skill in the art after review of the foregoing figuresand description.

To aid the Patent Office and any readers of any patent issued on thisapplication in interpreting the claims appended hereto, applicants wishto note that they do not intend any of the appended claims or claimelements to invoke 35 U.S.C. § 112(f) unless the words “means for” or“step for” are explicitly used in the particular claim.

The invention claimed is:
 1. A switching driver for driving a transducercomprising: first and second supply nodes for connection to first andsecond voltage supplies defining an input voltage; an output bridgestage comprising a first output switch connected between a firstswitching voltage node and an output node and a second output switchconnected between a second switching voltage node and the output node; acapacitor; a network of switches connecting said first and second supplynodes with said first and second switching voltage nodes and saidcapacitor, the network of switches being operable in different switchstates to provide different voltages at the first and second switchingvoltage nodes, said switch states comprising: a first switch state inwhich the capacitor is connected between the first and second supplynodes to be charged to the input voltage and the first and secondswitching voltage nodes are coupled to the first and second supply nodesrespectively; and a second switch state in which the second switchingvoltage node is connected to a voltage different to that at the secondsupply node and the capacitor is connected between the second switchingvoltage node and the first switching voltage node to provide a boostedvoltage at the first switching node; and a controller configured tocontrol the switch state of the network of switches and a duty cycle ofthe first and second output switches of the output bridge stage based onan input signal to generate an output signal at the output node fordriving the transducer; wherein, in the second switch state, the secondswitching voltage node is connected to the first supply node; andwherein the network of switches is further operable in a third switchstate in which the first switching voltage node is connected to thesecond supply node and said capacitor is connected between the secondsupply node and the second switching voltage node to provide a boostedvoltage at the second switching node.
 2. The switching driver of claim 1wherein the first voltage supply is more positive that the secondvoltage supply and the controller is configured to operate: in the firstswitch state in a first mode of operation to provide a drive signal atthe output node in a range between the first and second voltagesupplies; in the second switch state in second mode of operation toprovide a drive signal at the output node in a range between the firstvoltage supply and the first voltage supply boosted positively by theinput voltage; and in the third switch state in a third mode ofoperation to provide a drive signal at the output node in a rangebetween the second voltage supply and the second voltage supply boostednegatively by the input voltage.
 3. The switching driver of claim 1wherein the network of switches is configured such that, in use, avoltage difference across any of the switches of the network of switchesand the first and second output switches is not substantially greater inmagnitude than the input voltage.
 4. The switching driver of claim 1wherein the network of switches is configured such that a firstelectrode of the capacitor can be selective connected to either of thefirst or second supply nodes.
 5. The switching driver of claim 4 whereinthe network of switches is configured such that a second electrode ofthe capacitor can be selectively connected to either of the first orsecond supply nodes.
 6. The switching driver of claim 1 wherein thecapacitor is connected between the first and second switching voltagenodes in parallel with the output bridge stage.
 7. A switching driverfor driving a transducer comprising: first and second supply nodes forconnection to first and second voltage supplies defining an inputvoltage; an output bridge stage comprising a first output switchconnected between a first switching voltage node and an output node anda second output switch connected between a second switching voltage nodeand the output node; a capacitor; a network of switches connecting saidfirst and second supply nodes with said first and second switchingvoltage nodes and said capacitor, the network of switches being operablein different switch states to provide different voltages at the firstand second switching voltage nodes, said switch states comprising: afirst switch state in which the capacitor is connected between the firstand second supply nodes to be charged to the input voltage and the firstand second switching voltage nodes are coupled to the first and secondsupply nodes respectively; and a second switch state in which the secondswitching voltage node is connected to a voltage different to that atthe second supply node and the capacitor is connected between the secondswitching voltage node and the first switching voltage node to provide aboosted voltage at the first switching node; and a controller configuredto control the switch state of the network of switches and a duty cycleof the first and second output switches of the output bridge stage basedon an input signal to generate an output signal at the output node fordriving the transducer; wherein the capacitor is connected between thefirst and second switching voltage nodes in parallel with the outputbridge stage; and wherein the network of switches comprises for drivinga transducer comprising: a first switch connecting the first supply nodeto a first supply select node; a second switch connecting the secondsupply node to the first supply select node; a third switch connectingthe first supply select node to the first switching voltage node; afourth switch connecting the first supply node to a second supply selectnode; a fifth switch connecting the second supply node to the secondsupply select node; and a sixth switch connecting the second supplyselect node to the second switching voltage node.
 8. The switchingdriver of claim 7 wherein the controller is configured such that, whenoperating in the second switch state: the fourth switch and sixth switchare closed, with the fifth switch open, to connect the first supply nodeto the second switching voltage node; and the second and third switchesare open, with the first switch closed, so as to disconnect the firstswitching voltage node from the first and second supply nodes and tolimit the voltage difference across any of the first to third switchesto be not substantially greater than the input voltage.
 9. The switchingdriver of claim 1 wherein said capacitor has a capacitance which islarge enough to store sufficient charge to supply the transducer througha cycle of the input signal.
 10. A switching driver for driving atransducer comprising: first and second supply nodes for connection tofirst and second voltage supplies defining an input voltage; an outputbridge stage comprising a first output switch connected between a firstswitching voltage node and an output node and a second output switchconnected between a second switching voltage node and the output node; acapacitor; a network of switches connecting said first and second supplynodes with said first and second switching voltage nodes and saidcapacitor, the network of switches being operable in different switchstates to provide different voltages at the first and second switchingvoltage nodes, said switch states comprising: a first switch state inwhich the capacitor is connected between the first and second supplynodes to be charged to the input voltage and the first and secondswitching voltage nodes are coupled to the first and second supply nodesrespectively; and a second switch state in which the second switchingvoltage node is connected to a voltage different to that at the secondsupply node and the capacitor is connected between the second switchingvoltage node and the first switching voltage node to provide a boostedvoltage at the first switching node; and a controller configured tocontrol the switch state of the network of switches and a duty cycle ofthe first and second output switches of the output bridge stage based onan input signal to generate an output signal at the output node fordriving the transducer; wherein said capacitor is a first capacitor andthe switching driver further comprises a second capacitor, wherein: inat least one of the first and second switch states, the second capacitoris connected between the first and second voltage supplies to be chargedto the input voltage; in the second switch state, the second switchingvoltage node is connected to the first supply node; and the switchnetwork is further operable in a third switch state in which the secondcapacitor is connected between the first supply node and the secondswitching voltage node to provide a boosted voltage at the secondswitching node and the first capacitor is connected between the secondswitching voltage node and the first switching voltage node to provide afurther boosted voltage at the first switching voltage node.
 11. Theswitching driver of claim 10 wherein the first capacitor is connectedbetween the first and second switching voltage nodes in parallel withthe output bridge stage.
 12. The switching driver of claim 1 wherein atleast one of the switches of the network of switches and the first andsecond output switches comprises an NMOS transistor where at least partof the NMOS transistor is formed within an N-well in a substrate andwherein the switching driver is configured such that the N-well of theNMOS transistor is, in use, driven with a voltage based on the voltagesat the first and second switching voltage nodes.
 13. A switching drivercircuit comprising: a first switching driver for driving a transducer,the first switching driver comprising: first and second supply nodes forconnection to first and second voltage supplies defining an inputvoltage; an output bridge stage comprising a first output switchconnected between a first switching voltage node and an output node anda second output switch connected between a second switching voltage nodeand the output node; a capacitor; a network of switches connecting saidfirst and second supply nodes with said first and second switchingvoltage nodes and said capacitor, the network of switches being operablein different switch states to provide different voltages at the firstand second switching voltage nodes, said switch states comprising: afirst switch state in which the capacitor is connected between the firstand second supply nodes to be charged to the input voltage and the firstand second switching voltage nodes are coupled to the first and secondsupply nodes respectively; and a second switch state in which the secondswitching voltage node is connected to a voltage different to that atthe second supply node and the capacitor is connected between the secondswitching voltage node and the first switching voltage node to provide aboosted voltage at the first switching node; and a controller configuredto control the switch state of the network of switches and a duty cycleof the first and second output switches of the output bridge stage basedon an input signal to generate an output signal at the output node fordriving the transducer; and a second switching driver, the first andsecond switching drivers being configured to drive the transducer in abridge-tied load configuration.
 14. The switching driver circuit ofclaim 13 wherein the second switching driver comprises an output bridgestage, a capacitor and a network of switches operable in the same way asthe first switching driver and wherein the controller is configured tocontrol the switch state of the network of switches and duty cycle ofthe output bridge stage of both the first and second switching driversbased on an input signal to generate a differential output signal.
 15. Aswitching driver circuit comprising the switching driver of claim 1 anda DC-DC converter configured to receive at least one input voltagesupply and to generate at least one of said first and second voltagesupplies.
 16. A switching driver circuit comprising the switching driverof claim 1 and an inductor connected to the output node for connectionin series with the transducer.
 17. A switching driver circuit comprisingthe switching driver of claim 1 and said transducer.
 18. An electronicdevice comprising the switching driver of claim 1.